A go-around is an aborted landing of an aircraft that is on final approach.
The term arises from the traditional use of traffic patterns at airfields. A landing aircraft will first join the circuit pattern and prepare for landing in an orderly fashion. If for some reason the pilot decides not to land, the pilot can simply fly back up to circuit height, and complete another circuit. The term go-around is still used even for modern airliners, though they may not use traditional circuit patterns for landing.
Initiation of a go-around procedure may be either ordered by air traffic control (normally the local or 'tower' controller in a controlled field) or decided by the pilot in command of the aircraft.
At a towered field, the local controller may instruct the pilot to go around if there is an unsafe condition such as an aircraft, vehicle, or object on the runway. The pilot in command may decide to go around at any time, for example, if the aircraft is not lined up or configured properly for a safe landing; an aircraft, vehicle or other object has not cleared the runway; no landing clearance was received (at a towered field); the landing gear is not properly extended; a dangerous meteorological condition is experienced on final approach (e.g., poor visibility, excessive cross-winds, windshear, etc.); excessive energy (too high or too fast); or any other unsafe condition is detected.
In naval aviation, the term wave-off is used instead of go-around. When touching down on an aircraft carrier, a pilot always initiates a wave-off by applying full thrust as a fail-safe measure. That way, if the plane's tailhook fails to catch any of the arrestor cables (known as a (deck) "bolter") the aircraft can climb again. If the tailhook catches a cable, the aircraft will stop in short order regardless. Conversely, if a wave-off were not initiated and the aircraft were not arrested, it would not have enough power and/or runway to fly off the carrier safely.