A transmission from one station to another station in circumstances where two-way communication cannot be established but where it is believed that the called station is able to receive the transmission.
ICAO, Official definition
ANNEX 10/II Aeronautical Telecommunications — Communication Procedures including those with PANS status 2001, 6th ed. (Amdt 78, 14/07/2003)
PANS-ATM Procedures for Air Navigation Services — Air Traffic Management [Doc 4444] 2001, 14th ed. (Amdt 3, 29/06/2004)