UTC is the three-letter abbreviation for Coordinated Universal Time, the world’s primary time standard based on atomic time.
UTC is generated from International Atomic Time (TAI), a scale built from measurements by hundreds of atomic clocks at laboratories around the world. It is then adjusted with leap seconds so that it stays within about 0.9 seconds of UT1, a time scale based on Earth’s irregular rotation.
UTC became the official international abbreviation in 1967. The letter order is a deliberate compromise between the English name, Coordinated Universal Time, and the French temps universel coordonné; neither language’s natural initials were chosen. This is why the abbreviation is UTC rather than CUT or TUC.
GMT is a related but older term based historically on mean solar time at Greenwich. In everyday use, GMT and UTC often indicate the same zero-hour offset, but UTC is the modern scientific and civil standard. Time zones are written as offsets from it, such as UTC+5:30 for India.