hairline crack:

A tiny stress crack which forms due to strains in the material or extreme temperature differences; as opposed to crazing, a single crack of this type will often occur alone
halftrack: vehicle with caterpillar tracks over the rear wheels to provide motive power but steered by normal front wheels
halogen: One of the chemical elements fluorine, chlorine, bromine, iodine, or astatine.
halogen bulb: A bulb containing a trace of a halogen, such as iodine. A halogen bulb gives off a brighter light.
hand cranked: Before Cadillac invented and produced electric starters, engines were started by means of a handle which was inserted into the front of the engine and rotated manually. After 1930 it became obsolete.
handbrake: A brake operated by a hand lever. It may also refer to the park brake.
handbrake turn: 180 turn achieved by applying the handbrake (acting on the rear wheels) hard when the vehicle is starting to turn
handcrafted: Something that is built by hand rather than in an assembly line (i.e., mass produced).
Hands-free: A car cell phone that allows the driver to dial and talk on the phone without handling the phone.
hard stop: Hard braking, but not necessarily with locked wheels
Hard-Dry: The condition of paint when it is hard enough to polish
Hardener: Chemical added to plastic filler to induce hardening as used in auto body repair.
Hardening: The process of paint, epoxy, or glue becoming hard. The drying or hardening of paint film goes through several stages. The first stage is called "dust-free"; at this stage, the paint has hardened sufficiently to prevent dust from becoming embedded in the paint film. The second stage is called "touch-dry"; at this point, the paint film can actually be touched with light finger pressure. The third and final stage is referred to as "hard-dry"; at this point, the paint film is hard enough to polish.
Harness: A bundle of electrical wires. For convenience in handling and for neatness, all wires going to certain part of the vehicle are bundled together into a harness
hatchback: [1] The rear opening of a vehicle which allows passage into its cab.

[2] A hatchback.

[3] A tailgate.

[4] An opening in a deck through which cargo and stores are loaded or unloaded.

head gasket: The gasket at the top of the cylinder and sits between the cylinder and the head. It keeps the coolant out of the cylinders and retains compression in the cylinder. Also called cylinder head gasket. Also see blown head gasket
head lamp: The main light on the front of a vehicle. In Britain, "headlamp" refers to the actual unit, whereas "headlight" is used for the unit as well as for its function and where emphasis is on the actual light produced by the lamp. In North America the terms are interchangeable.
 Headlights: a clear set of lights at the front of the car so the driver can see in the dark.
headliner: Fabric or vinyl upholstery on the interior of the roof of a vehicle. The British term is "headlining."
head-on collision: An accident in which two cars hit each other's front end. Also called "head-on crash."
head restraint: An extension on the upper portion of the seatback. Some are permanently attached and other are adjustable. They are designed to reduce whiplash injuries caused by sudden stopping or collisions. They limit the rearward motion of the head and neck. Some head restraints contain radio speakers or even TV sets.
heated windshield: Windshield with a heating element to facilitate defrosting (de-icing)
heater: A device which gives off heat. In automobiles, it heats the interior of the vehicle. In a water-cooled engine the coolant is channelled through the heater in the passenger compartment. Some cars used an electric or gasoline heater because they had no engine coolant (e.g., the Volkswagen Beetle).
hemi: see engine types
high gear: The top gear or the cruising gear of a transmission. It is usually the highest number, i.e., fifth gear of a five-speed transmission.
high gloss: very shiny, bright appearance
high-performance: producing better than average results
highway: Any public road outside the cities with a foundation and a hard surface. Originally, the highway was a "way higher than the ground," e.g., as opposed to stage coach tracks. Since similar speed limits exist in the USA and Canada for all types of highways, both single or multilane (usually between 50 and 70 mph or 80 and 110 kph), the terms "highway driving speeds" and "highway driving" should not be associated with speeds higher than 70 mph (110 kph).
honeycomb: A pattern of hexagonal shapes, like bees' cells.
hood: the sheet metal covering the engine

(Br) a convertible top

hood badge: An emblem reflecting the name or logo of the car manufacturer and is secured to the front end of the hood. Some are bolted or pasted flat to the front, others stand upright on fixed or pivoting rods.
horn: A device for sounding an alarm
Horsepower (hP): A measurement of the engine's ability to perform work. One horsepower is defined as the ability to lift 550 pounds, one foot, in one second. Therefore the higher the horsepower of the vehicle, the more mass it can move or the more powerful it is.
hose: A flexible tube used to convey liquid. In most automobiles, hoses connected independent components like the radiator, water pump, and heater.
hot rod: A production car that has been modified by the owner in the attempt to increase acceleration and top end speed. Although the term can be applied to any modified car, it is usually reserved for vehicles produced from 1930 to 1940's. Typically the engine is modified, and some body panels removed. Many were painted with a design of flames behind the front wheels to give the appearance that this vehicle was "hot" -- thus the name.
hot wire: [1] The positive wire coming from the battery or generating system.

[2] As a verb, it indicates the starting procedure when by-passing the ignition key and normal starting procedure.

hubcap: The covering that fits over the end of the wheel spindle to keep dust and water away from the wheel bearings and brakes. It is often a styling feature, but it also acts as a good container for keeping the lug nuts from being lost when changing tires. When a vehicle becomes stuck in the snow, it can also be used as a temporary shovel. Also called wheel cover.