\begin{definition}[Definition:Astronomical Distance Units/Parsec]
The '''parsec''' is a derived unit of length.
It is derived as the distance at which $1$ astronomical unit subtends an angle of $1$ second of arc.
It is therefore exactly $\dfrac {648 \, 000} \pi$ astronomical units.
The '''parsec''' is the standard unit of measurement used by astronomers when discussing distances.
\end{definition}