Definition of

Light-year

Light-year
The distance that light travels in a vacuum in one year.

It is about 9.46 trillion kilometers, or 9.46 x 1015 meters

Proxima Centauri, our nearest star (apart from the Sun), is about 4 light years away.

In other words: if we go at the speed of light (about 300,000 km per second) for just over 4 years we would cover the distance to it.

Note that light-year is not a measure of time.
See: Speed
 
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