Temps Atomique International, or International Atomic Time.
The measurement of TAI is based on the international standard (SI) second, which is defined in terms of atomic measurements. The time scale is calculated by the Bureau International des Poids et Mesures, which periodically averages readings taken from over 200 atomic clocks worldwide. It is estimated to drift by no more than one-tenth of a microsecond per year.
The epoch for TAI is the start of January 1, 1958; this is when it is defined as being in sync with astronomical time (UT1). Unlike coordinated universal time (UTC), TAI is not adjusted with leap seconds to keep it close to UT1. Thus, TAI is equal to UTC plus the sum total of all historical leap seconds. At this writing, the last leap second was on January 1, 1999, and TAI is ahead of UTC by 32 seconds. [Note (2002-11-18): This still holds true through the end of 2002.]
(Sources: Time Service Dept., US Naval Observatory; Bureau International des Poids et Mesures)