A name given to a person who carries a radio. This allows people on a radio network to indicate which people are talking with each other. A typical radio signal would be: "Union 1 to Union 4, what is your location? Over".
Name given to a specific civillian or military airplane to identify it from the other aircraft. Airlines usually go by the airline name while military squadrons have their own assigned names. Also given to pilots in the military by others and usually is a play on words or actions.