Definition of flag:
"captains park in the second row, flag officers next to the building."
a piece of cloth
, often decorated with an emblem
, used as a visual signal
; a flag flown by a ship to show the presence on board of the admiral
; the admiral himself, or his flagship
to mark with a flag, especially to indicate the importance of; (often with down) To signal to, especially to stop a passing vehicle etc
"please flag a taxi down for me."