Meaning

• A piece of cloth, often decorated with an emblem, used as a visual signal or symbol.

• An exact representation of a flag (for example: a digital one used in websites).

• A flag flown by a ship to show the presence on board of the admiral; the admiral himself, or his flagship.

• (often used attributively) A signal flag.

• The use of a flag, especially to indicate the start of a race or other event.

• A variable or memory location that stores a true-or-false, yes-or-no value, typically either recording the fact that a certain event has occurred or requesting that a certain optional action take place.

• In a command line interface, a command parameter requesting optional behavior or otherwise modifying the action of the command being invoked.

• A mechanical indicator that pops up to draw the pilot's attention to a problem or malfunction.

• The game of capture the flag.

• A sequence of faces of a given polytope, one of each dimension up to that of the polytope (formally, though in practice not always explicitly, including the null face and the polytope itself), such that each face in the sequence is part of the next-higher dimension face.

• A sequence of subspaces of a vector space, beginning with the null space and ending with the vector space itself, such that each member of the sequence (until the last) is a proper subspace of the next.

Meaning

• To furnish or deck out with flags.

• To mark with a flag, especially to indicate the importance of something.

• (often with down) To signal to, especially to stop a passing vehicle etc.

  "Please flag down a taxi for me."

• To convey (a message) by means of flag signals.

  "to flag an order to troops or vessels at a distance"

• (often with up) To note, mark or point out for attention.

  "I've flagged up the need for further investigation into this."

• To signal (an event).

  "The compiler flagged three errors."

• To set a program variable to true.

  "Flag the debug option before running the program."

• To decoy (game) by waving a flag, handkerchief, etc. to arouse the animal's curiosity.

• To penalize for an infraction.

  "The defender was flagged for unsportsmanlike conduct."

• To defeat (an opponent) on time, especially in a blitz game.