The principal meridian is the meridian of Earth which passes through the Royal Observatory in Greenwich, Greater London, England, which is used as the internationally-recognised $0 \degrees$ reference longitude meridian.
Also defined as
However, this definition is less convenient.
Also known as
In common parlance, a terrestrial meridian is usually referred to as just a meridian.
The terrestrial descriptor is added so as to distinguish it from a celestial meridian.
- 1933: D.M.Y. Sommerville: Analytical Conics (3rd ed.) ... (previous) ... (next): Chapter $\text I$. Coordinates: $2$. Coordinates
- 1976: W.M. Smart: Textbook on Spherical Astronomy (6th ed.) ... (previous) ... (next): Chapter $\text I$. Spherical Trigonometry: $4$. Terrestrial latitude and longitude.
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): meridian: 1.
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): meridian