An Acronym for Universale Temps Coordineé, ie Coordinated Universal Time.
This is the standard time zone all other time zones are based on. It replaced GMT in the early 1970s. UTC is measured with atomic clocks. It is kept within 0.9 seconds of TAI, but with an integral number of leap seconds added occasionally to reflect the season at and location on the planet. This way UTC stays in sync with the rotation of the earth, making sure midnight always occurs at the same time relative to the stars. To date all corrections have been positive to account for the tidal drag on the earth which slows the planet down.