a naval officer above the rank of captain, as a fleet admiral, admiral, vice-admiral, rear admiral, or commodore, who is entitled to display a flag indicating his or her rank.
2.
(formerly) an officer in command of a fleet, squadron, or group of ships.
Origin
1655-65
Examples from the web for flag officer
She has accommodations for a flag officer and staff.
To receive testimony on general and flag officer requirements.
British Dictionary definitions for flag officer
flag officer
noun
1.
an officer in certain navies of the rank of rear admiral or above and entitled to fly its flag