Universal Time

n. Greenwich Mean Time

Universal Time (UT) is a timescale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the meridian of Greenwich, England, which is the conventional zero meridian for geographic longitude. GMT is sometimes used as a synonym for UTC. The old GMT has been split, in effect, into UTC and UT1.
See more at Wikipedia.org...

Noun
1. the local time at the 0 meridian passing through Greenwich, England; it is the same everywhere
(synonym) Greenwich Mean Time, Greenwich Time, GMT, UT, UT1
(hypernym) time
(hyponym) coordinated universal time, UTC

1. The basis for coordinated dissemination of time signals, counted from 0000 at midnight. 2. In celestial navigation applications, the time which gives the exact rotational orientation of the Earth obtained from UTC by applying increments determined by the U.S. Naval Observatory. 3. A measure of time that conforms, within a close approximation, to the mean diurnal rotation of the Earth and serves as the basis of civil timekeeping. Note: Universal Time (UT1) is determined from observations of the stars, radio sources, and also from ranging observations of the Moon and artificial Earth satellites. The scale determined directly from such observations is designated Universal Time Observed (UTO); it is slightly dependent on the place of observation. When UTO is corrected for the shift in longitude of the observing station caused by polar motion, the time scale UT1 is obtained. When an accuracy better than one second is not required, Universal Time can be used to mean Coordinated Universal Time (UTC). [JP1] 4. The official civil time of the United Kingdom. Formerly called Greenwich Mean Time. Contrast with Coordinated Universal Time.

<time, standard> (UT) The mean solar time along the prime meridian (0 longitude) that runs through the Greenwich Observatory outside of London, UK, where the current system originated. UT is tied to the rotation of the Earth in respect to the fictitious "mean Sun".
Greenwich Mean Time (GMT) was measured from Greenwich mean midday until 1925 when the reference point was changed from noon to midnight and the name changed to "Universal Time".
There are three separate definitions, UT0, UT1, and UT2, depending on which corrections have been applied to the Earth's motion. Coordinated Universal Time is kept within 0.9 seconds of UT1, by addition of leap seconds to International Atomic Time.
(2001-08-02)