One-way communication from aircraft to stations or locations on the surface of the earth.
ICAO, Official definition,
PANS-ATM Procedures for Air Navigation Services — Air Traffic Management [Doc 4444] 2001, 14th ed. (Amdt 3, 29/06/2004)
ANNEX 10/II Aeronautical Telecommunications — Communication Procedures including those with PANS status 2001, 6th ed. (Amdt 78, 14/07/2003)
To be distinguished from air-ground communication !