A year is a unit of time, but a light-year is a unit of distance. Specifically, it is the distance that light travels in one year. Since light travels at 299,792 km/s and a year consists of 31.557 x 10⁶ s, a light-year is 9.4605 x 10¹² km or about 6 trillion miles.

The light-year is used to express distances to stars and galaxies.