first officer

Short definition from Wiktionary

Noun

1. 1
   The copilot of an aircraft.
2. 2
   The chief mate of a merchant ship.
3. 3
   The executive officer, second in command of a naval vessel.
4. 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. 5
   Air Transport Auxiliary rank corresponding to Flight Lieutenant in the Royal Air Force.
6. 6
   Women's Royal Naval Service rank corresponding to Lieutenant-Commander in the Royal Navy.