\begin{definition}[Definition:Meridian (Terrestrial)/Principal]
The '''principal meridian''' is the meridian of Earth which passes through the {{WP|Royal_Observatory,_Greenwich|Royal Observatory}} in {{WP|Greenwich|Greenwich}}, {{WP|Greater_London|Greater London}}, {{WP|England|England}}, which is used as the internationally-recognised $0 \degrees$ reference longitude meridian.
\end{definition}