Universal Time (UT) is a measurement of time based on the Earth's rotation. It is therefore influenced by its irregularities due, among other factors, to the tides produced by the Sun and Moon.
There are several Universal Times, one of the most important being Coordinated Universal Time (UTC). This international standard is the basis of civil time. It is derived from International Atomic Time (TAI), itself based on the atomic definition of the second and measured using a series of atomic clocks.
See "Watch with universal hours".