The full definition from
ICAO Annex 2, Rules of the Air is: “For IFR flights, the time at
which it is estimated that the aircraft will arrive over that designated point,
defined by reference to navigation aids, from which it is intended that an
instrument approach procedure will be commenced, or, if no navigation aid is
associated with the aerodrome, the time at which the aircraft will arrive over
the aerodrome.
For VFR flights, the time
which it is estimated that the aircraft will arrive over the aerodrome.”