NOUN (3)

1. a flag that serves as a warning signal;
- Example: "we didn't swim at the beach because the red flag was up"

2. the emblem of socialist revolution;

3. something that irritates or demands immediate action;
- Example: "doing that is like waving a red flag in front of a bull"

The Collaborative International Dictionary of English v.0.48:

Flag \Flag\, n. [Cf. LG. & G. flagge, Sw. flagg, Dan. flag, D.
   vlag. See Flag to hang loose.]
   1. That which flags or hangs down loosely.
      [1913 Webster]

   2. A cloth usually bearing a device or devices and used to
      indicate nationality, party, etc., or to give or ask
      information; -- commonly attached to a staff to be waved
      by the wind; a standard; a banner; an ensign; the colors;
      as, the national flag; a military or a naval flag.
      [1913 Webster]

   3. (Zool.)
      (a) A group of feathers on the lower part of the legs of
          certain hawks, owls, etc.
      (b) A group of elongated wing feathers in certain hawks.
      (c) The bushy tail of a dog, as of a setter.
          [1913 Webster]

   4. (Zool.) One of the wing feathers next the body of a bird;
      -- called also flag feather.
      [Webster 1913 Suppl.]

   Black flag. See under Black.

   Flag captain, Flag leutenant, etc., special officers
      attached to the flagship, as aids to the flag officer.

   Flag officer, the commander of a fleet or squadron; an
      admiral, or commodore.

   Flag of truse, a white flag carried or displayed to an
      enemy, as an invitation to conference, or for the purpose
      of making some communication not hostile.

   Flag share, the flag officer's share of prize money.

   Flag station (Railroad), a station at which trains do not
      stop unless signaled to do so, by a flag hung out or
      waved.

   National flag, a flag of a particular country, on which
      some national emblem or device, is emblazoned.

   Red flag, a flag of a red color, displayed as a signal of
      danger or token of defiance; the emblem of anarchists.

   To dip, the flag, to mlower it and quickly restore it to
      its place; -- done as a mark of respect.

   To hang out the white flag, to ask truce or quarter, or, in
      some cases, to manifest a friendly design by exhibiting a
      white flag.

   To hang the flag half-mast high or To hang the flag
   half-staff or To hang the flag at half-staff, to raise it
      only half way to the mast or staff, as a token or sign of
      mourning.

   To strike the flag or To lower the flag, to haul it down,
      in token of respect, submission, or, in an engagement, of
      surrender.

   Yellow flag, the quarantine flag of all nations; also
      carried at a vessel's fore, to denote that an infectious
      disease is on board.
      [1913 Webster]

WordNet (r) 3.0 (2006):

red flag
    n 1: a flag that serves as a warning signal; "we didn't swim at
         the beach because the red flag was up"
    2: the emblem of socialist revolution
    3: something that irritates or demands immediate action; "doing
       that is like waving a red flag in front of a bull"

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "red flag":
   Roman candle, aid to navigation, alarm, amber light, balefire,
   beacon, beacon fire, bell, bell buoy, blinker, blue peter, buoy,
   caution light, danger sign, early symptom, falling barometer,
   flare, fog bell, fog signal, fog whistle, foghorn,
   gathering clouds, glance, go light, gong buoy, green light,
   heliograph, high sign, international alphabet flag,
   international numeral pennant, kick, leer, marker beacon, nod,
   nudge, omen, parachute flare, pilot flag, poke, police whistle,
   precursor, preliminary sign, premonitory sign, premonitory symptom,
   prodroma, prodrome, quarantine flag, radio beacon, red light,
   rocket, sailing aid, semaphore, semaphore flag,
   semaphore telegraph, sign, signal, signal beacon, signal bell,
   signal fire, signal flag, signal gong, signal gun, signal lamp,
   signal light, signal mast, signal post, signal rocket, signal shot,
   signal siren, signal tower, skull and crossbones, spar buoy,
   stop light, storm petrel, stormy petrel, symptom, the nod,
   the wink, thundercloud, thunderhead, touch, traffic light,
   traffic signal, warning sign, warning signal, watch fire,
   white flag, wigwag, wigwag flag, wink, yellow flag, yellow jack