identification friend or foe

Short definition from Wiktionary

Noun

1. 1
   An identification system designed for command and control. It enables military and national (civilian air traffic control) interrogation systems to identify aircraft, vehicles or forces as friendly and to determine their bearing and their range from the interrogator: In a civilian aircraft: a transponder that responds by sending a code identifying itself.
2. 2
   An identification system designed for command and control. It enables military and national (civilian air traffic control) interrogation systems to identify aircraft, vehicles or forces as friendly and to determine their bearing and their range from the interrogator: In a military aircraft: a transponder that responds by sending a similarly coded signal, only when detecting a code indicating the r