Other forms: light years
In astronomy, a light year is the distance that light travels in a year. Informally, astronomers use the light year to describe distances between planets, stars, moons and other astronomical bodies.
Light years are sometimes mistakenly thought of as units of time — because of the year — but they're actually units of distance, equal to about six trillion miles. In 1851, the light year was first used for defining distances by a German astronomer who compared it to a "hiking hour," or the distance a person can hike in one hour. The term can also figuratively mean "a long way:" "Our team is light years ahead of where they were last summer!"