light-year I. \ˈ ̷ ̷| ̷ ̷\noun Etymology:light (I) : a unit of length in interstellar astronomy equal to the distance that light travels in one year in a vacuum or 5,878,000,000,000 miles II. noun : an extremely large measure of comparison (as of distance, time, or quality) < two minutes and yet light-years away from the crowded village — Suzanne Patterson >