A light year is the distance that light travels through a vacuum in one year.

A light-year (or light year, abbreviation: ly) is a unit of length used informally to express astronomical distances. It is well over 9 trillion kilometres (or about 6 trillion miles). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year. Because it includes the word year, the term light-year is sometimes misinterpreted as a unit of time.