Definition:Astronomical Distance Units/Light Year

Definition
The light year is a derived unit of length.

It is derived as the distance travelled by light in a vacuum over the course of $1$ Julian year ($365.25$ days).

It is defined as being exactly $9 \, 460 \, 730 \, 472 \, 580 \, 800$ metres.

The light year is the standard unit of measurement used by astronomers when discussing distances between stars and galaxies, particularly in popular and non-scientific literature.

Serious astronomers use the term parsec instead.