Proper noun
Geocentric Coordinate Time
[1983–] A coordinate time standard intended to be used as the independent variable of time for all calculations pertaining to precession, nutation, the Moon, and artificial satellites of the Earth, equivalent to the proper time experienced by a clock at rest in a coordinate frame co-moving with the center of the Earth.
The present definition of TT is a linear scaling of Geocentric Coordinate Time (TCG), which is the proper time of a notional observer who is infinitely far away (so not affected by gravitational time dilation) and at rest relative to Earth. Source: Internet
TT was formally defined in terms of Geocentric Coordinate Time (TCG), defined by the IAU on the same occasion. Source: Internet