What is the distance that light travels in one year called?

The distance that light travels in one year is referred to as a light year. This term specifically quantifies astronomical distances based on the speed of light, which is approximately 299,792 kilometers per second (or about 186,282 miles per second). When scientists measure distances in space, using a unit like the light year allows for a more practical understanding of vast distances because space is so immense. For instance, when we say that a star is 4 light years away from Earth, it means that the light we see from that star today actually left it 4 years ago.

In contrast, a parsec is another unit of distance used in astronomy, equivalent to about 3.26 light years, and an astronomical unit (AU) is a measurement based on the average distance from the Earth to the Sun, roughly 93 million miles (or about 150 million kilometers). The term "units of distance" is too vague and doesn’t specify any related astronomical measurement. By distinguishing the unique distance light travels in a vacuum over the course of a year, the term light year provides a clear and powerful tool for discussing the enormity of the universe.