Definition of flag:
"captains park in the second row, flag officers next to the building."
(n) :
a piece of
cloth, often decorated with an
emblem, used as a visual
signal or
symbol; a flag flown by a ship to show the presence on board of the
admiral; the admiral himself, or his flagship
(v) :
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."