A person on duty during a 'watch' — a fixed block of hours when someone must stay awake and in charge while everyone else rests. The word belongs mainly to ships. On a ship, the day is cut into watches of about four hours, and the watchkeeper is the officer or crew member responsible during those hours: steering, reading the instruments, looking out for other ships, and waking the captain if something is wrong. So it is a job title with real responsibility, not just 'a guard'. In the navy an officer may be 'watchkeeping certified', meaning they are qualified to be left in charge of a moving ship at night. Outside ships you will meet it occasionally for anyone keeping a similar shift — in a power station, a radio room, or a hospital operations room — and sometimes loosely for a guard or a lookout. It is a formal, technical, slightly old-fashioned word; ordinary people talking about the man at the gate of a building would say 'security guard' or 'watchman', not 'watchkeeper'.

值班人員、（船上的）值更人員／當值船副