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.”