An Acronym for Temps Atomique International, ie International Atomic Time.

This is a time zone measured with atomic clocks. Its reading increases monotonically, without leap seconds added. TAI is defined as the unit of duration of 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of Caesium 133.

TAI was exactly 31s ahead of UTC on Jul 01, 1997. There were no leap seconds on that day.

Compare GMT, UTC.