The prime meridian is the meridian (line of longitude) passing through Royal Greenwich Observatory, Greenwich, England; it is the meridian at which the longitude is 0 degrees.

The meridian was agreed upon in October 1884. At the behest of the President of the United States of America, 41 delegates from 25 nations met in Washington, D.C, USA for the International Meridian Conference.

At the conference the following important principles were established:

  1. It was desirable to adopt a single world meridian to replace the numerous ones already in existence.
  2. The meridian passing through the principal transit instrument at the Observatory at Greenwich was to be the "initial meridian".
  3. That all longitude would be calculated both east and west from this meridian up to 180°.
  4. All countries would adopt a universal day.
  5. The universal day would be a mean solar day, beginning at the mean midnight at Greenwich and counted on a 24-hour clock.
  6. That nautical and astronomical days everywhere would begin at mean midnight.
  7. All technical studies to regulate and extend the application of the decimal system to the division of time and space would be supported. 
Resolution 2, fixing the meridian at Greenwich, was passed 22-1 (San Domingo voted against); France and Brazil abstained.

The international date line is on the opposite side of the world to the prime meridian.

Previously, the meridian of the Canarian island El Hierro was used by most of the navies of continental Europe.
It was conventionally defined as the meridian 20 degrees west from Paris.