ICAO aircraft type designator, a three- or four-character alphanumeric code designating every aircraft type (and some sub-types) that may appear in flight planning
ICAO aircraft marshalling signals, visual signalling between ground personnel and pilots on an airport, aircraft carrier or helipad
ICAO 24-bit address, allocation of 24 bit addresses to states to uniquely identify aircraft worldwide
ICAO altitude code aka Gillham code, an encoding used in older aircraft to report altitudes