Abbreviated UTC. Used to be GreenwichMeanTime, GMT.

''UTC is coordinated by different observatories on and over the planet, correcting the braking affects of the Moon over Earth's rotation, the difference between sidereal time and all that gang of differences of times due to the quasi-elliptical orbit of our planet, etc... GMT nor UTC have the concept of leap seconds, which are inserted once a year if needed...''

AKA Zulu Time, abbreviated Z, from the association of time zones with letters of the [NATO/ITU/FAA] phonetic alphabet. 

A map of all time zones can be found at: http://aa.usno.navy.mil/AA/faq/docs/world_tzones.html.
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The reason why UTC is the acronym for UniversalCoordinatedTime:
(from http://www.boulder.nist.gov/timefreq/general/misc.htm#Anchor-14550)

''In 1970 the Coordinated Universal Time system was devised by an international advisory group of technical experts within the International Telecommunication Union (ITU). The ITU felt it was best to designate a single abbreviation for use in all languages in order to minimize confusion. Since unanimous agreement could not be achieved on using either the English word order, CUT, or the French word order, TUC, the acronym UTC was chosen as a compromise.''