What is Greenwich Mean Time?

Answer 1

Greenwich Mean Time (GMT)
GMT is a mathematical mean, defined in terms of the solar second, measured at the former location of the Royal Observatory in England, located on the Prime Meridian (zero degrees longitude). GMT is useful for navigation (when converted to UT1, which is outside the scope of this document), but not for time-keeping programs.
When speaking loosely, GMT and Coordinated Universal Time (UTC) are roughly the same, but careful speakers will note that GMT is based on the solar second, while UTC is based on the standard second. The delta between the two time systems is too small for humans to notice, but very important for computer programs and universal synchronization.

Answer 2

Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory, Greenwich in England. It is now often used to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone, although strictly UTC is an atomic time scale which only approximates GMT in the old sense. It is also used to refer to Universal Time (UT), which is the astronomical concept that directly replaced the original GMT.