High intensity discharge (HID) lamps are also known as xenon lamps.

HID lamps create light using an electric arc inside a glass tube containing xenon gas.

Size for size, Xenon lamps produce more light than conventional incandescent lamps, so manufacturers can fit smaller lamps for the same output.

A hypoid bevel gear has a special curved tooth form which allows the pinion to be mounted lower than the centre of the crown wheel. When this is used in a final drive unit, it lowers the line of the propshaft.

Horsepower can refer generally to the power output of an engine, or it can refer to one of several specific measurement units.

Brake horsepower (bhp) has long been the standard UK/US unit of engine power. 1bhp = 745.7 watts.

Pferdestärke (PS) is 'metric horsepower', defined using the SI system but very nearly the same as bhp. 1PS = 735.5 watts.

The RAC horsepower rating is very different: it was a taxation system which rated engines using a calculation which was intended to reflect engine power, though in practice it was too simplistic to be very accurate.

Tappets or valve lifters are a link in the valvetrain between the camshaft and the valve. Hydraulic tappets expand under engine oil pressure, taking up slack and automatically adjusting for wear in the valvetrain.

Hydraulic tappets do not need regular maintenance like solid tappets but require clean oil and can be unreliable at very high engine speeds.


A hatchback body has an opening tailgate at the back to provide access to the luggage area.

Usually a hatchback is a three or five door family car, though many different types of vehicle can have a similar rear tailgate arrangement.

Hatchback is often abbreviated to hatch, as in hot hatch.

See also fastback, notchback.