In aviation, the first officer (FO) is the second pilot (also 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" and the other the "pilot not flying", or "pilot monitoring", 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.
Many airlines promote by seniority only within their own company. As a consequence, an airline first officer may be older and/or have more flight experience than a 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 ("right seat") and the left-hand side of a helicopter (the reason for this difference is related to, in many cases, the pilot flying being unable to release the right hand from the cyclic control to operate the instruments, thus they sit on the right side and do that with the left hand). Other airlines may designate the more senior of two first officers operating a long-haul sector together with a captain in an enlarged crew as the senior first officer. The senior first officer will then sit in the left seat when the captain takes a rest.