Longman Dictionary of Contemporary English
Wiktionary
n. A rank in the RAF, between that of pilot officer and flight lieutenant, equivalent to an Army lieutenant or Royal Navy sublieutenant
Wikipedia
Flying officer (Fg Off in the RAF and IAF; FLGOFF in the RAAF; FGOFF in the RNZAF; formerly F/O in all services and still frequently in the RAF) is a junior commissioned rank in the Royal Air Force and the air forces of many countries which have historical British influence. It is also sometimes used as the English translation of an equivalent rank in countries which have a non-English air force-specific rank structure. In these cases a Flying Officer usually ranks above pilot officer and immediately below flight lieutenant.
It has a NATO ranking code of OF-1 and is equivalent to a lieutenant in the British Army or the Royal Marines. However, it is superior to the nearest equivalent rank of sub-lieutenant in the Royal Navy.
The equivalent rank in the Women's Auxiliary Air Force was "section officer".
Usage examples of "flying officer".
Lieutenant Howard and the rest of the airplane crew were there: the pilot, who was a flying officer, and the navigator, who was a sergeant, and the gunner, who was a corporal.
The controller lifted one headphone, and caught the information that Flying Officer Harris was sick and would not be reporting for the first shift the next day.
That was one of the inexpressible things about being a military flying officer.