\begin{definition}[Definition:Astronomical Distance Units/Light Year]
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.
\end{definition}