A headlight is a bright lamp mounted on the front of a vehicle to illuminate the road ahead in darkness or poor visibility. Cars, trucks, motorcycles, and bicycles all use headlights so drivers can see where they're going at night and so other people can see them coming.

Most vehicles have two headlights, one on each side of the front. They can usually switch between high beams (very bright, for dark country roads with no other traffic) and low beams (dimmer, for city streets and highways where bright lights might blind oncoming drivers). Modern headlights use different technologies: older ones use halogen bulbs, while newer ones might use LEDs or high-intensity discharge lamps that shine brighter while using less energy.

Before electric headlights were invented in the early 1900s, cars used gas lamps or even candles, which barely lit the road at all. If someone says they were “caught like a deer in the headlights,” they mean they froze in surprise or fear, unable to react, like a deer that stands still when a car's headlights suddenly shine on it at night.