The Most Detailed Dictionary of Automotive Terms

How to use this dictionary

Find a term in its system group, or decode an acronym in the table near the end. The main unit conversions are below. Longer entries are in our glossary, engine data in the engines section.

Engine and fuel terms

Displacement (engine capacity): the swept volume of all cylinders, in cm³ or litres.

Compression ratio: largest cylinder volume divided by the smallest. Petrol engines typically run about 8:1 to 12:1, direct-injection diesels about 14:1 to 23:1.

Torque: twisting force at the crankshaft, in N·m (lbf-ft); power is torque times rotational speed.

Naturally aspirated: an engine breathing at atmospheric pressure, with no turbocharger or supercharger.

Turbocharger: an exhaust-driven turbine spinning a compressor that forces in extra air.

Supercharger: a compressor driven by a belt or electric motor; it responds instantly but uses engine power.

Intercooler: a heat exchanger that cools compressed intake air so denser air reaches the cylinders.

Direct injection: fuel sprayed straight into the combustion chamber, allowing higher compression.

Common rail: the modern diesel injection system, in which one high-pressure rail feeds electronically controlled injectors.

Timing belt or chain: links crankshaft and camshafts; a belt has a fixed replacement interval, a chain can stretch.

Dual-mass flywheel: a two-piece, spring-damped flywheel that absorbs vibration; on many diesels it is replaced with the clutch.

Octane rating (RON): resistance to knock. European pumps show RON (95 or 98); US pumps show a lower-looking average of RON and MON.

Auxiliary heater: a fuel-fired unit that warms coolant and cabin before a cold start; see how Webasto liquid heaters work.

Transmission and drivetrain terms

Clutch: the friction coupling between engine and gearbox; dry clutches run in air, wet ones in oil.

Torque-converter automatic: an automatic with a fluid coupling and planetary gearsets. More in our article on ZF gearboxes.

CVT: a continuously variable transmission; a belt or chain between two variable pulleys gives continuously changing ratios.

DCT (DSG, PDK): a dual-clutch gearbox with one clutch for odd and one for even gears, so the next gear is preselected.

AMT (automated manual): a manual gearbox whose clutch and shifts are worked by actuators.

Differential: gears that let the left and right wheels turn at different speeds in a corner.

Limited-slip differential (LSD): limits that speed difference so drive is not wasted on a spinning wheel.

Transfer case: the 4x4 gearbox that splits drive between axles, often with a low-range ratio.

FWD, RWD, AWD, 4x4: front-, rear- or all-wheel drive; 4x4 usually implies an off-roader with low range.

Chassis, suspension and body terms

Wheelbase: distance between front and rear wheel centres; longer usually means more cabin space.

Track: distance between left and right wheel centres on one axle.

Monocoque (unibody) and body-on-frame: a monocoque shell carries the loads itself; body-on-frame vehicles sit on a separate ladder frame.

MacPherson strut: compact front suspension combining spring and damper in one strut above a lower arm.

Shock absorber (damper): controls spring bounce; leaking or worn dampers commonly fail roadworthiness tests.

Anti-roll bar: a torsion bar linking left and right wheels to reduce body lean.

Camber, toe and caster: wheel-alignment angles; wrong settings cause uneven tyre wear and pulling.

Kerb weight (US: curb weight), GVW and payload: weight ready to drive, maximum laden weight, and the difference.

Braked and unbraked towing capacity: the heaviest trailer allowed with and without trailer brakes, as stated in the vehicle documents.

Bonnet and boot (US: hood and trunk): the engine cover and the luggage compartment.

Brakes, tyres and wheels terms

ABS: anti-lock braking stops the wheels locking so the car stays steerable; the pedal pulses when it works.

EBD and brake assist: electronic brake-force distribution balances braking between axles; brake assist adds full pressure in an emergency stop.

Brake fade: loss of braking when pads, discs or fluid overheat, typically on long descents.

Regenerative braking: the electric motor acts as a generator when slowing, recharging the battery; friction brakes are still needed.

Tyre size (205/55 R16): 205 mm width, sidewall height 55% of width, radial construction, 16-inch rim.

Load index and speed rating: the code after the size, such as 91V. Both must match the vehicle documents.

DOT date code: four digits giving week and year of manufacture: 3524 means week 35 of 2024.

3PMSF: the three-peak mountain snowflake symbol, proof of a certified snow test under UN Regulation 117; M+S alone is a self-declaration. See our winter tyres guide.

Tread depth: depth of the main grooves; the EU legal minimum is 1.6 mm (0.06 in).

TPMS: tyre-pressure monitoring with wheel sensors (direct) or via ABS wheel-speed signals (indirect).

Alloy wheel: a cast or forged aluminium-alloy wheel, lighter than steel; see choosing wheels for looks, price or reliability.

Safety and driver-assistance terms

Many of these systems are now mandatory: the EU General Safety Regulation (GSR2) applies to vehicles first registered from 7 July 2024.

ESC (ESP): electronic stability control brakes individual wheels when the car starts to skid. EU rules required it on new car types from 1 November 2011.

Traction control (TCS, ASR): limits wheel spin by braking the spinning wheel or reducing power.

ISOFIX: the standard child-seat mounting: two rigid lower anchors 280 mm (11 in) apart plus a top tether or support leg.

Euro NCAP: Europe's crash-test programme, rating new cars up to five stars.

AEB: autonomous emergency braking brakes automatically if a collision is imminent.

ACC: adaptive cruise control holds a set gap to the vehicle ahead using radar or a camera.

Lane keeping (LKA, ELKS): warns about or corrects a drift out of lane; emergency lane keeping is a GSR2 item.

ISA: intelligent speed assistance reads signs and map data and warns about speeding.

DDAW: driver drowsiness and attention warning, which suggests a break.

eCall: dials 112 automatically after a serious crash and sends the car's position; required for new car types approved in the EU since 31 March 2018.

Electrics, electronics, hybrids and EV terms

12 V battery: starts the engine and powers the electrics; hybrids and EVs have one too. It caused 45.4% of call-outs in ADAC's 2026 breakdown statistics (2025 data).

ECU: electronic control unit; a modern car has dozens.

CAN bus: the network, introduced by Bosch in 1986, that lets control units share data.

OBD (EOBD): on-board diagnostics via a 16-pin socket, required in the EU on petrol cars first registered from 2001 and diesels from 2004. Fault codes begin with P, B, C or U.

Check-engine light (MIL): the malfunction indicator lamp, lit when an emissions-related fault is stored.

Trip computer: shows consumption, range and average speed; see on-board computer functions. In a 2020 ADAC test of 80 cars, only about a quarter showed consumption accurately.

Mild hybrid (MHEV): a 48 V starter-generator assists the engine but cannot drive the car electrically on its own.

Full hybrid (HEV): drives short distances electrically and charges itself; the Toyota Prius, on sale from December 1997, made it mainstream.

Plug-in hybrid (PHEV): a larger, grid-charged battery. An ICCT study (September 2026) found real-world CO2 of 2021 registrations averaged 3.5 times the official figure.

BEV and kWh: a battery-electric vehicle has no combustion engine; battery capacity is given in kWh, consumption in kWh/100 km.

AC and DC charging, CCS: AC charging goes through the on-board charger, DC fast charging bypasses it; CCS is the European combined connector.

Emissions, regulations and paperwork terms

Catalytic converter: converts carbon monoxide, hydrocarbons and nitrogen oxides into carbon dioxide, water and nitrogen.

EGR: exhaust gas recirculation returns some exhaust to the intake, lowering combustion temperature and NOx.

DPF: a diesel particulate filter traps soot and burns it off above about 600 °C (1,112 °F); frequent short trips can clog it.

SCR and AdBlue: selective catalytic reduction injects AdBlue, a 32.5% urea solution, turning NOx into nitrogen and water. It freezes at about -11 °C (12 °F).

Euro 6 and Euro 7: EU emission stages. Euro 6 limits NOx to 80 mg/km for diesels and 60 mg/km for petrol cars; Euro 7 is due for new types from November 2026.

WLTP and NEDC: WLTP replaced NEDC in 2017 and 2018. Its 30-minute cycle covers 23.25 km (14.4 miles) at up to 131.3 km/h (81.6 mph).

VIN: the 17-character vehicle identification number, without the letters I, O or Q; the first three characters identify the manufacturer.

Type approval and CoC: type approval certifies a model meets EU rules; the Certificate of Conformity ties an individual car to it.

Roadworthiness test (MOT, HU): the periodic inspection, called the MOT in the UK and HU in Germany.

Common abbreviations at a glance