- noun a line of longitude situated at 0° and passing through Greenwich, England
- The line of 0° longitude on the surface of a planet. In the case of the Earth, a line running through Greenwich in London is accepted as the prime meridian, and in recent decades the equivalent points have also been nominated on Mercury, Mars and Venus (longitude) and on many solar system satellites. The equator and poles of a rotating body are decided by nature, but the prime meridian has to be decided arbitrarily.
- The meridian which has an assigned longitude of 0°. The Greenwich meridian is generally considered as the prime meridian of the planet earth. Also called zero meridian.