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Liz Potter
Written by Liz Potter


Greenwich Mean Time: the time at Greenwich, in England, used as an international standard

View the full definition in the Macmillan Dictionary.

Origin and usage

GMT is an initialism, short for Greenwich Mean Time. It was first used in the mid 19th century, while the full form dates from the late 18th century.


When the clocks go back in autumn in the UK, the country returns to GMT or Greenwich Mean Time until the following March. GMT is the time at the meridian in Greenwich in southeast London, home of the Royal Observatory. The Greenwich meridian was adopted as the zero of longitude at an international conference in 1884, making it the basis for time throughout the world. This prime meridian was marked at the time by a metal strip in the ground, and it can still be seen today, although it is now made of stainless steel rather than the original brass. Although the main time standard in the world is now Coordinated Universal Time, the term Greenwich Mean Time, usually shortened to GMT, remains in general use, especially in the UK and some Commonwealth countries. It is used by public bodies such as the BBC, the Met Office and the Royal Navy.


“The Royal Observatory, home of Greenwich Mean Time and the Prime Meridian lines is one of the most important historic scientific sites in the world.”

Related words

standard time, time zone, DST

Browse related words in the Macmillan Thesaurus.

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Liz Potter

Liz Potter

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