Dictionary

light-year

light–year

noun \ˈlīt-ˌyir\

: a unit of distance equal to the distance that light travels in one year (about 5.88 trillion miles or 9.46 trillion kilometers)

light-years —used to say that someone or something is much better or more advanced than others

—used to refer to a time that is or seems very far away

Full Definition of LIGHT-YEAR

1
:  a unit of length in astronomy equal to the distance that light travels in one year in a vacuum or about 5.88 trillion miles (9.46 trillion kilometers)
2
:  an extremely large measure of comparison (as of distance, time, or quality) <seems like light–years ago> <has light–years more talent> <two minutes and yet light–years away from the crowded village — Suzanne Patterson>

First Known Use of LIGHT-YEAR

1888

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