A word in the right gear.
Short definitions for unfamiliar terms. Every entry connects to a fuller explanation with sources.
120 terms
- Adaptive cruise control (ACC)
A function that adjusts vehicle speed to maintain a selected following gap from a detected vehicle ahead, within its operating limits. It does not remove the driver’s responsibility to monitor and act.
Read the full explanation- Aerodynamic drag
The aerodynamic force opposing motion relative to air. Its size depends on air conditions, relative speed, reference area, and a drag coefficient.
Read the full explanation- All-season tire
A tire intended to cover a range of seasonal conditions. The label does not guarantee the same snow or cold-weather capability as a dedicated winter design.
Read the full explanation- All-wheel drive (AWD)
A drive system that can send propulsion to both the front and rear wheels. How and when it does this depends on the design.
Read the full explanation- Alternating-current (AC) charging
Charging in which the car receives alternating current and its onboard charger converts that supply to direct current for the traction battery.
Read the full explanation- Alternator
An engine-driven generator that supplies electrical energy in a conventional vehicle charging system.
Read the full explanation- Anti-Knock Index (AKI)
The average of Research and Motor octane numbers, often shown as (R+M)/2 on North American gasoline pumps. It is a different scale from RON.
Read the full explanation- Anti-lock braking system (ABS)
A system that monitors wheel rotation and modulates brake pressure to help prevent wheels locking during braking.
Read the full explanation- Automatic emergency braking (AEB)
A collision-intervention function that can apply or supplement braking when a detected collision is imminent. Detection coverage and operating conditions depend on the system.
Read the full explanation- Automatic engine stop-start
A control function that stops a combustion engine during permitted pauses and restarts it when required. It differs from a starter button and does not by itself make a car a hybrid.
Read the full explanation- Battery electric vehicle (BEV)
A vehicle driven by an electric motor using energy from a battery charged from an external electricity supply.
Read the full explanation- Battery state of health
An assessment of battery condition relative to a stated reference, often involving retained energy capacity. The metric, test method, and baseline must be identified before comparing results.
Read the full explanation- Bore
The inside diameter of an engine cylinder. Bore and stroke together determine swept volume.
Read the full explanation- Brake fluid
The specified liquid that transmits pressure through a hydraulic braking system. Different fluid types are not automatically interchangeable.
Read the full explanation- Brake pad
A friction component clamped against a brake disc to resist rotation. Its usable thickness must meet the applicable specification.
Read the full explanation- Brake shoe
A curved component carrying friction material inside a drum brake. It presses against the drum’s inner surface during braking.
Read the full explanation- Cabin air filter
A filter in the passenger ventilation system that catches particles such as dust and pollen. Its construction and replacement requirements vary.
Read the full explanation- Camshaft
An engine shaft with shaped lobes that control when valves open and close.
Read the full explanation- Catalytic converter
An exhaust treatment device whose catalyst helps chemical reactions convert particular pollutants into less harmful substances. It does not remove every emission.
Read the full explanation- Charging curve
A record or illustration of how charging power changes over a session, commonly plotted against battery state of charge. Its test conditions and measurement point matter.
Read the full explanation- Clutch
A mechanism that connects or separates rotating parts. A typical manual-car clutch uses a clamped friction disc between the engine and transmission.
Read the full explanation- Combustion
Burning: a chemical reaction that releases energy. In an engine, it creates hot gas that can do mechanical work.
Read the full explanation- Compression ratio
For geometric engine compression ratio, the cylinder’s largest volume above the piston divided by its smallest volume. It differs from displacement and from pressure measured in a compression test.
Read the full explanation- Continuously variable transmission (CVT)
A transmission that can vary its ratio without fixed steps. A common design uses adjustable pulleys and a belt; hybrid e-CVT systems can differ.
Read the full explanation- Coolant
A fluid used to transfer heat and help keep components within their intended temperature range. Follow the vehicle’s exact specification.
Read the full explanation- Corrosion
Deterioration of a material through reactions with its environment. In vehicle metalwork it can reduce thickness or strength; its location and extent determine its significance.
Read the full explanation- Crankshaft
The rotating engine shaft linked to the pistons by connecting rods. It turns their repeated strokes into rotation.
Read the full explanation- Crash avoidance
Measures intended to help prevent a collision or reduce its severity, including some warning and automatic-intervention systems. This differs from protection during an impact.
Read the full explanation- Crash protection
How a vehicle’s structure and restraint systems aim to reduce injury during a collision. Test results describe specific assessed conditions, not every possible crash.
Read the full explanation- Cylinder
The bore in which an engine piston moves. Gas pressure in the cylinder pushes the piston during the power stroke.
Read the full explanation- Damper (shock absorber)
A suspension component that restrains repeated bouncing. Springs and dampers perform different jobs.
Read the full explanation- Depreciation
The reduction in a vehicle’s value over time. In a simple ownership comparison, purchase price minus resale proceeds represents the value lost over the period; future resale value is an assumption.
Read the full explanation- Diagnostic trouble code (DTC)
A stored identifier for a condition detected by a vehicle control system. It helps direct diagnosis; it does not by itself prove which part needs replacement.
Read the full explanation- Diesel exhaust fluid (DEF)
A specified urea-and-water solution used in many SCR exhaust systems, also sold under the AdBlue name. It belongs in its separate designated tank, never the fuel tank.
Read the full explanation- Diesel particulate filter (DPF)
An exhaust filter that captures diesel soot. Controlled oxidation removes accumulated soot during regeneration; servicing requirements depend on the system.
Read the full explanation- Differential
A drive mechanism that allows connected outputs, commonly the left and right driven wheels, to rotate at different speeds. Designs differ in how they manage unequal grip.
Read the full explanation- Direct injection
Fuel injection directly into the combustion chamber, rather than into an intake passage before the intake valve.
Read the full explanation- Direct-current (DC) charging
Charging in which external equipment supplies direct current to the traction battery through the vehicle’s charging controls. Compatibility and accepted power depend on the exact car.
Read the full explanation- Displacement
The total volume swept by all engine pistons in one stroke. Often stated in liters or cubic centimeters.
Read the full explanation- Distance-record discrepancy
An inconsistency between recorded vehicle distances, dates, or units. It needs investigation; the discrepancy alone does not establish its cause.
Read the full explanation- Drag coefficient (Cd)
A dimensionless number used with a specified reference area to describe aerodynamic drag. Comparing coefficients alone omits vehicle size.
Read the full explanation- Dual-clutch transmission (DCT)
A transmission using two controlled clutches and separate gear paths, allowing a likely next gear to be prepared before the active path changes.
Read the full explanation- Electric power steering (EPS)
Steering assistance supplied by a controlled electric motor. Many systems retain a mechanical steering connection; the term alone does not mean steer-by-wire or automated driving.
Read the full explanation- Electronic stability control (ESC)
A system that compares the driver’s steering input with vehicle motion and can apply individual brakes, reduce drive power, or both to help correct unwanted motion.
Read the full explanation- Engine
Here, an internal combustion engine: a machine that burns fuel with air and converts some of the released energy into mechanical work.
Read the full explanation- Engine air filter
A filter that removes particles from air entering an engine. It is separate from the cabin ventilation filter.
Read the full explanation- Engine oil
Lubricant that helps reduce friction between moving engine surfaces. The required grade and performance specification depend on the engine.
Read the full explanation- Engine thermostat
A temperature-control valve that regulates coolant flow toward the radiator. Its operation and control depend on the engine design.
Read the full explanation- Final drive
A drive reduction between the transmission output and the driven wheels in a conventional driveline. Its ratio combines with the selected transmission ratio to set the overall speed relationship.
Read the full explanation- Flood damage
Vehicle damage associated with flooding or substantial water exposure. Effects can involve hidden systems and may develop after the visible water has gone.
Read the full explanation- Flywheel
A rotating mass connected to the engine crankshaft. In a typical manual car it also provides a surface against which the clutch disc is clamped.
Read the full explanation- Front-wheel drive (FWD)
A drive layout that sends propulsion to the front wheels. It does not mean only the front wheels have brakes.
Read the full explanation- Fuel injector
A controlled device that meters and sprays fuel. Its location and operating pressure depend on the injection system.
Read the full explanation- Gear ratio
The relationship between input and output rotation in a gear system. Ratios trade speed for turning force.
Read the full explanation- Glow plug
An electrically heated element used in many diesel engines to support starting and combustion. It supplies heat rather than a gasoline-style ignition spark.
Read the full explanation- Gross vehicle weight rating (GVWR)
The manufacturer’s maximum permitted loaded vehicle weight. It is a rating, not the vehicle’s measured current weight.
Read the full explanation- Heat pump
Equipment that uses energy to move heat from one place to another. Some cars use it for cabin heating and other temperature-control tasks.
Read the full explanation- Horsepower (hp)
A unit of power. One mechanical horsepower is about 746 watts. Metric horsepower is a slightly different unit.
Read the full explanation- Hybrid
A car that combines an internal combustion engine with electric drive. A conventional hybrid does not plug into an external charger.
Read the full explanation- Ignition coil
An electrical component that produces the high voltage needed to make a spark cross a spark plug gap.
Read the full explanation- Intercooler or charge-air cooler
A heat exchanger that removes heat from compressed intake air. It may transfer heat to outside air or to a separate cooling-liquid circuit, depending on the design.
Read the full explanation- Kilowatt (kW)
A unit of power equal to 1,000 watts. It measures a rate of energy transfer, not stored battery capacity.
Read the full explanation- Kilowatt-hour (kWh)
A unit of energy. One kilowatt of power supplied for one hour transfers one kilowatt-hour. Battery capacity and electricity consumption use energy units.
Read the full explanation- Lane centering assistance
A function that provides continuing steering assistance to help center the vehicle in a detected lane. It differs from a lane-departure warning, which alerts the driver.
Read the full explanation- Load-bearing structure
Parts of the vehicle body or frame that carry forces and support components. Damage near important mountings can matter even when the visible affected area is small.
Read the full explanation- Lock-up clutch
A clutch in many torque converters that creates a more direct mechanical connection under suitable conditions to reduce slip and associated losses.
Read the full explanation- Miles per gallon (mpg)
Distance traveled per gallon of fuel. Higher is more fuel-efficient under the same conditions. Check whether the gallon is a US or Imperial gallon.
Read the full explanation- Miles per gallon equivalent (MPGe)
Distance traveled using energy equal to the energy in one US gallon of gasoline. This compares energy efficiency, not the price of different fuels.
Read the full explanation- Minimum brake disc thickness
The specified minimum usable thickness for a particular brake disc. It is a component-specific limit, not a universal value for every car.
Read the full explanation- Misfire
A combustion event that fails to occur properly. Ignition, fuel delivery, and mechanical problems can cause it; it is not a diagnosis of one failed part.
Read the full explanation- Octane rating
A standardized measure of gasoline’s resistance to knock. The number must be read with the rating method, such as RON or AKI.
Read the full explanation- Odometer
The instrument that displays a vehicle’s accumulated distance. A reading alone does not establish mechanical condition.
Read the full explanation- Oil performance specification or approval
A stated set of oil-performance requirements from an industry body or vehicle manufacturer. It is separate from the viscosity grade and must match the engine’s instructions.
Read the full explanation- On-board diagnostics (OBD)
Vehicle computer functions that monitor specified engine and emissions systems, store fault information, and can request a warning light when a monitored problem is detected.
Read the full explanation- Ownership cost
The costs associated with buying, keeping, using, and eventually disposing of a vehicle over a stated period. The included categories and assumptions must be clear for comparisons to be useful.
Read the full explanation- Payload
Load carried by a vehicle, including occupants and cargo under the manufacturer’s definition. Added equipment and trailer tongue load can reduce the remaining allowance.
Read the full explanation- Piston
A part that slides inside an engine cylinder. A connecting rod links it to the crankshaft.
Read the full explanation- Plug-in hybrid electric vehicle (PHEV)
A hybrid whose traction battery can also charge from an external electricity supply. It still has a combustion engine.
Read the full explanation- Pre-purchase inspection
An independent professional assessment before buying. Scope, access, and testing limits should be agreed in advance.
Read the full explanation- Pulley
A rotating wheel-shaped component that guides a belt or similar drive element. Adjustable CVT pulleys change the belt’s effective running radius.
Read the full explanation- Radiator
A heat exchanger that transfers heat from circulating engine coolant to air. It is separate from the air-conditioning condenser.
Read the full explanation- Readiness monitor
A diagnostic test status showing whether an emissions-system check has completed. An unready monitor has not yet supplied a completed test result.
Read the full explanation- Rear-wheel drive (RWD)
A drive layout that sends propulsion to the rear wheels. Steering is normally still performed by the front wheels.
Read the full explanation- Refrigerant
A working fluid that circulates through a refrigeration system to transfer heat. It is different from engine coolant.
Read the full explanation- Regenerative braking
Using an electric motor as a generator while slowing down, recovering some of the car’s motion as electrical energy.
Read the full explanation- Research Octane Number (RON)
An octane rating determined under the Research test method. It is the pump-label method used in many markets.
Read the full explanation- Revolutions per minute (rpm)
A measure of rotational speed. Engine specifications use it to show the speed at which an output is produced.
Read the full explanation- Rim diameter
The wheel diameter at the tire’s seating area, reflected in the tire size marking. It is not the outside diameter of the complete tire-and-wheel assembly.
Read the full explanation- Safety recall
A campaign to address a safety-related defect or failure to meet applicable safety requirements. The notice identifies affected vehicles, the issue, and the required action; coverage and processes depend on the market.
Read the full explanation- Selective catalytic reduction (SCR)
An exhaust treatment process that reduces nitrogen oxides using a catalyst and a reducing substance. Many vehicle systems dose diesel exhaust fluid upstream.
Read the full explanation- Service history
Records of maintenance and repairs, ideally with dates, distance readings, and details identifying the vehicle and work done.
Read the full explanation- Severe-service conditions
Operating conditions that a manufacturer identifies as requiring different or additional maintenance. The definition and affected tasks are vehicle-specific.
Read the full explanation- Spark plug
A component that produces the spark used to start combustion in a spark-ignited engine.
Read the full explanation- Starter motor
An electric motor that turns a combustion engine during starting in a conventional starting system.
Read the full explanation- State of charge
How full the traction battery is within the vehicle’s operating range, normally displayed as a percentage. It describes the current charge level, not battery health.
Read the full explanation- Stroke
One trip of a piston between its highest and lowest positions. Also the distance traveled during that trip.
Read the full explanation- Suspension
The system that supports the car while allowing controlled movement between the wheels and body over uneven ground.
Read the full explanation- Suspension spring
A component that supports load while allowing controlled suspension deflection. Coil, leaf, and air springs are different designs.
Read the full explanation- Suspension strut
A suspension component that provides damping and also forms a structural part of wheel location and support. A complete strut assembly may include a spring and mount.
Read the full explanation- Temporary spare tire
A spare intended for restricted temporary use under stated vehicle and tire limits. It is not automatically equivalent to the normally fitted tires.
Read the full explanation- Thermal management
Controlling component temperatures through heating, cooling, and operating limits. A vehicle may manage separate temperatures for its battery, motor, power electronics, engine, and cabin.
Read the full explanation- Timing drive
The mechanism that keeps crankshaft and camshaft rotation coordinated. It may use a toothed belt, a chain, or another design.
Read the full explanation- Tire aspect ratio
The tire sidewall height expressed as a percentage of its nominal section width. In 205/55 R16, 55 is the aspect ratio, not a height in millimeters.
Read the full explanation- Tire load index
A code that corresponds to a tire’s load-carrying capacity under specified conditions. Match the vehicle manufacturer’s requirements rather than choosing by size alone.
Read the full explanation- Tire mobility kit
Equipment, often sealant and an inflator, intended to provide temporary mobility for specified tire damage. It cannot address every type of tire failure.
Read the full explanation- Tire pressure monitoring system (TPMS)
A system that warns about tire inflation problems. It does not replace regular pressure checks with a gauge or an inspection for damage.
Read the full explanation- Torque
A turning effect around a shaft or axis. Common units are newton-meters (N·m) and pound-feet.
Read the full explanation- Torque converter
A fluid-based drive coupling with an impeller, turbine, and stator. It can multiply torque under some operating conditions and may include a lock-up clutch.
Read the full explanation- Traction
The grip between a tire and the road that lets the tire transmit forces for acceleration, braking, and turning.
Read the full explanation- Traction battery
The battery that stores electrical energy used to propel an electric or hybrid vehicle. It is distinct from the low-voltage auxiliary battery.
Read the full explanation- Trailer tongue load
The downward load a trailer applies at the tow vehicle’s coupling. It contributes to the tow vehicle’s loading and must satisfy the relevant limits.
Read the full explanation- Transmission
The system that transfers mechanical power toward the wheels and establishes a useful relationship between input and output speed.
Read the full explanation- Turbocharger
An exhaust-driven turbine linked to a compressor that raises the pressure of air supplied to the engine.
Read the full explanation- Usable battery capacity
The battery energy made available for use by the vehicle’s controls. Confirm that a quoted capacity is usable energy before using it in a range calculation.
Read the full explanation- Vehicle history report
A compilation of reported vehicle events from stated databases. Coverage and reporting delays vary; an absent event does not prove it never happened.
Read the full explanation- Vehicle identification number (VIN)
The identifier assigned to a vehicle. Use it to match records and check applicable recall information; lookup coverage varies by market.
Read the full explanation- Viscosity
A fluid’s resistance to flow under stated conditions. Engine-oil viscosity changes with temperature, and grades describe standardized test behavior.
Read the full explanation- Wheel alignment
Checking and, where adjustable, setting wheel geometry to the vehicle manufacturer’s specification.
Read the full explanation- Wheel balancing
Measuring and correcting mass imbalance in a rotating wheel and tire assembly, usually with a balancing machine and small weights.
Read the full explanation- Winter tire
A tire designed for cold-weather conditions, including snow. Its suitability still depends on the exact product, approved specification, condition, and local requirements.
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