Definition
A person assigned to perform duties in an aircraft during flight time. In FAA usage, this includes flight crewmembers (pilots, flight engineers, flight navigators) and other crewmembers such as flight attendants or aircraft loadmasters. A crewmember is on duty for the operation of the aircraft, not simply a passenger riding along.
Plain English
Anyone with an official job to do on the aircraft while it's flying — the pilots and anyone else assigned to operate or serve the flight. Passengers are not crewmembers.
Context Anchor
Seen in medical certificate rules when deciding whether a person may legally serve in a required role on a flight.
Derivation
From 'crew' (a group assigned to a shared task, originally a ship's working company) and 'member' (one of a group). The aviation use carries over directly from maritime usage, where the crew were the people working the vessel as opposed to the passengers being carried.
Why Pilots Care
Only crewmembers are required to hold medical certificates appropriate to their duties; passengers are exempt.
Intuition Check
Crewmember does not mean every person inside the airplane. It means a person assigned to do a crew duty for that flight.
Example Sentence 1
Each required crewmember must use supplemental oxygen above certain cabin altitudes.
Example Sentence 2
Each crewmember on the flight must possess a current medical certificate matching their assigned role.