Definition of Flag
A national standard on which are certain emblems; an ensign; a banner. It is carried by soldiers, ships, etc., and commonly displayed at forts, businesses and many other suitable places.
In common parlance, the word "flag,” when used as denoting a signal, does not necessarily mean the actual use of a flag, but by figure of speech the word is used in the secondary sense and signifies a signal given as with a flag, that is to say, as by a waving of the hand for the purpose of communicating information.
That's the definition of Flag in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.