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Definition: light year
The distance light travels in one year, which is roughly 6 trillion miles or 9.5 trillion kilometers. Abbreviated "ly," light years are used to measure distances in the universe. For example, our galaxy is presumed to be about 100 ly across. Proxima Centauri, at about 4.22 ly from the sun, is the closest star to the sun.
It's About Distance, Not Time
Because the word "year" is used, light years are often erroneously used to measure time. The phrase "I spent light years doing that" might sound triumphant, but it is like saying "I spent trillions of miles doing that."