The abbreviation GMT stands for Greenwich Mean Time or Greenwich Meridian Time. The Prime Meridian, or 0 degrees longitude, passes close to the Royal Observatory at Greenwich village near London. This is taken as the reference for calculating time zones east or west of the Prime Meridian.
GMT is also sometimes referred to as UTC, or Coordinated Universal Time, but the two are slightly different. In 1884, at the Washington Meridian Conference, GMT was adopted as the standard for calculating time all over the world. Universal Time and the idea of 24-hour time zone systems grew from GMT. When the sun is exactly overhead at the Prime Meridian, it is 12:00 noon at Greenwich, England.