first officer

first officer (plural first officers)

1. The copilot of an aircraft.
2. The chief mate of a merchant ship.
3. The executive officer, second in command of a naval vessel.
4. The most senior civil servant in the United States Foreign Service, with responsibility for running an embassy under the authority of and reporting to the ambassador.
5. Air Transport Auxiliary rank corresponding to Flight Lieutenant in the Royal Air Force.
6. Women's Royal Naval Service rank corresponding to Lieutenant-Commander in the Royal Navy.