The standard three-letter abbreviation for Greenwich Mean Time is GMT.
GMT originally referred to mean solar time at the Royal Observatory in Greenwich, London. Because Greenwich was associated with the zero-degree meridian, mariners used it as a reference for calculating longitude, and the term gradually became a worldwide timekeeping reference.
The abbreviation is formed directly from the three initials in “Greenwich Mean Time.” It is commonly used for the UTC+00:00 time zone, although UTC is the more precise modern standard for international timekeeping. GMT itself can have historical or astronomical meanings that depend on context.
A frequent mix-up is treating GMT and UTC as perfectly interchangeable in technical work. They are normally close enough for everyday communication, but UTC is maintained using atomic clocks and can differ slightly from Earth-rotation-based UT1. In the United Kingdom, GMT is also the legal winter-time designation, while summer uses British Summer Time.