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Terrestrial Time (TT) is a time standard (or type of time standard) defined for describing astronomical observations from Earth, that allows simple arithmetic to determine time intervals that are precise to the millisecond level and beyond. Some astronomical observations require that precision, e.g., for observations of moving planets and stars and observations of pulsar pulses and glitches. The J2000.0, B1950.0, and B1900.0 epochs are defined in terms of Terrestrial Time.
Such an Earth-based time standard is non-trivial and Terrestrial Time is the latest International Astronomical Union (IAU) version of one. Other (types of) time standards are also in use that have goals other than "time as measured on Earth": observations on Earth are complicated by its orbit and the finite speed of light: light from a nearby star viewed from Earth will reach Earth later by minutes if Earth is currently at the point of its orbit further from the star (Roemer delay), and the timing of actions off Earth make more sense if the effect of such delays is eliminated. Among the alternatives to Terrestrial Time are times counted as per at the Sun or at the solar system's barycenter.
Terrestrial Time uses the SI second, counting them from a point in time chosen to match that of other standards, but since then, time values based upon the various standards drift compared to each other. For standards based upon other celestial objects, this drift is a consequence of relativity. Some other Earth-based standards are more oriented to our everyday life, with occasional adjustments to remain in sync with the solar day: Terrestrial Time explicitly does not do this, merely counting the seconds that have passed.
The current method of tracking Terrestrial Time is International Atomic Time (TAI), a time standard based upon averaging a considerable number of atomic clocks. Geocentric Coordinate Time (TCG) is a related time standard: a theoretical time counting SI seconds at the point of the Earth's center of mass, but as if the Earth were not there, i.e., no effect of Earth's gravity. It is used to describe Earth-centric orbits and timing such as that of the Moon and artificial satellites.