Universal Time

Astronomically defined timescale that has superseded Greenwich Mean Time in scientific use. It is based on the rotation of the Earth, which is not quite constant. The term embraces several timescales, incorporating various kinds of correction for irregularities in the Earth's movements. The civil time used around the globe is UTC, Coordinated Universal Time, which is based on uniform atomic time, but which is corrected by the occasional addition or removal of ‘leap seconds’ to keep it in step with UT. See time.