Greenwich Mean Time (GMT) is the mean solar time at the Greenwich Observatory in Greenwich near London, England, which by convention is at 0 degrees geographic longitude. Theoretically, noon Greenwich Mean Time is the moment when the Sun crosses the meridian (and reaches its highest point in the sky. Because of the Earth's uneven speed in its elliptic orbit, this event may be up to 16 minutes off (known as the analemma); but this averages out over the year. So GMT follows a fictituous "mean sun" that moves at uniform speed along the equator in a year, and that appears to move across the sky in a day as the Earth rotates around its axis. In fact, time was more accurately measured by observations of stars crossing the meridian, which indicates sidereal time. A conventional formula then yields GMT.
GMT, mean solar days, and all clocks based on the rotation of the Earth have been abandoned, because the rotation of the Earth is somewhat irregular (see Delta-T). Nowadays, GMT has been replaced by UTC, which is measured by atomic clocks, but is kept within 0.9 seconds from GMT.