copilot n : a relief pilot on an airplane
In commercial aviation, the first officer is the second pilot (sometimes referred to as the "co-pilot") of an aircraft. The first officer is second-in-command of the aircraft, to the Captain who is the legal commander. In the event of incapacitation of the captain, the first officer will assume command of the aircraft.
Control of the aircraft is normally shared equally between the first officer and the captain, with one pilot normally designated the "Pilot Flying" (PF) and the other the "Pilot Not Flying" (PNF) for each flight. Even when the first officer is the flying pilot, however, the captain remains ultimately responsible for the aircraft, its passengers, and the crew. In typical day-to-day operations, the essential job tasks remain fairly equal.
Because many airlines promote by seniority only within their own company, the first officer may at times have more flying hours than the captain, by virtue of having experience from other airlines or the military. Traditionally, the first officer sits on the right-hand side of a fixed-wing aircraft and the left-hand side of a helicopter.
In the rank of Senior First Officer the pilot will also sit in the right hand seat. Often the Senior First Officer position is used within airlines to mean someone who has passed all the requirements for Captain, but there are no Captains positions within the company as yet, and therefore they are "on hold" until a position as captain becomes available when they will receive their Command Line Check
copilot in German: Erster Offizier (Luftfahrt)
copilot in Finnish: Perämies
copilot in Chinese: 副機師