: the mean time that is associated with the prime meridian of Greenwich, England, and historically used as the prime basis of standard time throughout the world
The British traditionally used Greenwich Mean Time, calculated from the meridian running through Greenwich, a borough of London.—
The New York Times
The New York TimesCoordinated Universal Time is what used to be called Greenwich Mean Time, but the new name doesn't make it any more universal than it ever was, and it remains a time zone centered in Britain.—Natalie Angier
—abbreviation GMT
Natalie Angiercalled also Greenwich Time
compare coordinated universal timeExamples of Greenwich Mean Time in a Sentence
Recent Examples on the Web
But about half a century after Big Ben was built in London, marking the center of Greenwich Mean Time, Albert Einstein’s theory of general relativity changed the way that scientists think about time.
—Joshua Rapp Learn, Discover Magazine, 31 July 2025
After a year went by, Luke took Sarah to Greenwich in South East London, where Greenwich Mean Time, the yearly average (or ‘mean’) of the time each day when the Sun crosses the Prime Meridian at the Royal Observatory Greenwich is kept.
—Erin Clack, People.com, 29 July 2025
The Royal Observatory Greenwich is home to Greenwich Mean Time and the Prime Meridian, an imaginary line of longitude, designated as 0 degrees that runs from the North Pole to the South Pole.
—Jamie Carter, Forbes.com, 10 July 2025
This new 'Greenwich Mean Time' (GMT) quickly spread and was adopted during the 1884 Meridian Conference as the new global time zone system, making the observatory the global reference center for time, navigation and astronomy.
—Anthony Wood, Space.com, 22 June 2025
Yesterday afternoon, at 13:25 Greenwich Mean Time, the International Space Station finished re-orienting itself in preparation to dock with an unmanned Russian resupply ship.
—IEEE Spectrum, 2 Aug. 2012
For the first time, there will also be a rebroadcast of each afternoon and evening’s programming at 7 a.m. Greenwich Mean Time.
—Eric Torres, Pitchfork, 25 Apr. 2025
From Pan Am to Space Speeding across the skies at several hundred miles per hour, pilots and air crews needed a system to track both local and Greenwich Mean Time, the Prime Meridian (as decided in 1884), and the standard time of air traffic controllers in the 1950s.
—Oren Hartov, Robb Report, 28 Mar. 2025
As such, the ISS operates on Coordinated Universal Time, UTC, also known as Greenwich Mean Time (GMT).
—Josh Dinner, Space.com, 8 Feb. 2025
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Joshua Rapp Learn, Discover Magazine, 31 July 2025
After a year went by, Luke took Sarah to Greenwich in South East London, where Greenwich Mean Time, the yearly average (or ‘mean’) of the time each day when the Sun crosses the Prime Meridian at the Royal Observatory Greenwich is kept.
—Erin Clack, People.com, 29 July 2025
The Royal Observatory Greenwich is home to Greenwich Mean Time and the Prime Meridian, an imaginary line of longitude, designated as 0 degrees that runs from the North Pole to the South Pole.
—Jamie Carter, Forbes.com, 10 July 2025
This new 'Greenwich Mean Time' (GMT) quickly spread and was adopted during the 1884 Meridian Conference as the new global time zone system, making the observatory the global reference center for time, navigation and astronomy.
—Anthony Wood, Space.com, 22 June 2025
Yesterday afternoon, at 13:25 Greenwich Mean Time, the International Space Station finished re-orienting itself in preparation to dock with an unmanned Russian resupply ship.
—IEEE Spectrum, 2 Aug. 2012
For the first time, there will also be a rebroadcast of each afternoon and evening’s programming at 7 a.m. Greenwich Mean Time.
—Eric Torres, Pitchfork, 25 Apr. 2025
From Pan Am to Space Speeding across the skies at several hundred miles per hour, pilots and air crews needed a system to track both local and Greenwich Mean Time, the Prime Meridian (as decided in 1884), and the standard time of air traffic controllers in the 1950s.
—Oren Hartov, Robb Report, 28 Mar. 2025
As such, the ISS operates on Coordinated Universal Time, UTC, also known as Greenwich Mean Time (GMT).
—Josh Dinner, Space.com, 8 Feb. 2025
Cite this Entry
“Greenwich Mean Time.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/Greenwich%20Mean%20Time. Accessed 10 Aug. 2025.
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Greenwich mean time
noun
Green·wich mean time
ˈgrin-ij- 
ˈgren-,
-ich-
: the time of the meridian of Greenwich used as the basis of standard time throughout the world
called also Greenwich time
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