Astronomy. the distance traversed by light in one mean solar year, about 5.88 trillion mi. (9.46 trillion km): used as a unit in measuring stellar distances.
Abbreviation: lt-yr.
2.
light-years.
a very great distance, especially in development or progress:
The new computer is light-years ahead of the old one.
a very long time:
It's been light-years since I've seen my childhood friends.
Origin
1885-90
light-year in Science
light-year The distance that light travels in a vacuum in one year, equal to about 9.46 trillion km (5.88 trillion mi). Light-years are used in measuring interstellar and intergalactic distances. Compare astronomical unit, parsec.