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.
a very great distance, esp. in development or progress: The new computer is light-years ahead of the old one.
b.
a very long time: It's been light-years since I've seen my childhood friends.