1.  red flag(n = noun.communication) - a flag that serves as a warning signal; "we didn't swim at the beach because the red flag was up"
is a kind of alarm, alarum, alert, warning signal, flag, signal flag

2.  red flag(n = noun.communication) - the emblem of socialist revolution;
is a kind of allegory, emblem

3.  red flag(n = noun.act) - something that irritates or demands immediate action; "doing that is like waving a red flag in front of a bull"
is a kind of annoyance, annoying, irritation, vexation

N  warning, early warning, caution, caveat, notice, premonition, premonishment, prediction, contraindication, lesson, dehortation, admonition, monition, alarm, handwriting on the wall, mene mene tekel upharsin, red flag, yellow flag, fog-signal, foghorn, siren, monitor, warning voice, Cassandra, signs of the times, Mother Cary's chickens, stormy petrel, bird of ill omen, gathering clouds, clouds in the horizon, death watch, watchtower, beacon, signal post, lighthouse, sentinel, sentry, watch, watchman, watch and ward, watchdog, bandog, housedog, patrol, patrolman, vedette, picket, bivouac, scout, spy, spial, undercover agent, mole, plainclothesman, advanced guard, rear guard, lookout, cautiousness, monitor, guard camera, radar, AWACS, spy satellite, spy-in-the- sky, U2 plane, spy plane, warning, premonitory, monitory, cautionary, admonitory, admonitive, sematic, warned, forewarned, on one's guard, in terrorem, Int, beware!, ware!, take care!, look out!, fore (golf), mind what you are about!, take care what you are about!, mind!, ne reveillez pas le chat qui dort, don't wake a sleeping cat, foenum habet in cornu, caveat actor, le silence du people est la legon des rois, verbum sat sapienti, a word to the wise is sufficient, un averti en vaut deux.

N  alarm, alarum, larum, alarm bell, tocsin, alerts, beat of drum, sound of trumpet, note of alarm, hue and cry, fire cross, signal of distress, blue lights, war-cry, war- whoop, warning, fogsignal, foghorn, yellow flag, danger signal, red light, red flag, fire bell, police whistle, false alarm, cry of wolf, bug-bear, bugaboo, alarming, Int, sauve qui peut!, every man for himself!, qui vive?.

alizarin red, alizarine red, american flag, american red elder, american red plum, american red squirrel, battle flag, black flag, blue flag, brick red, canadian red pine, capital of red china, cherry red, chrome red, code flag, confederate flag, congo red, dark red, deep red, eastern red cedar, european red elder, flag, flag captain, flag day, flag down, flag of truce, flag officer, flag rank, flag smut, flag smut fungus, flag stop, flag waving, forest red gum, giant red paintbrush, in the red, indian red, japanese red army, japanese red pine, knight of the square flag, little red riding hood, myrtle flag, national flag, nautical signal flag, northern red oak, orange red, oxblood red, paint the town red, palmetto flag, pilot flag, pirate flag, pompeian red, purplish red, red, red admiral, red alder, red alert, red algae, red amaranth, red angel's trumpet, red army faction, red ash, red baiting, red baneberry, red bat, red bay, red bearberry, red beech, red birch, red bird's eye, red blood cell, red blooded, red bone marrow, red bordeaux, red brass, red brigades, red bryony, red buckeye, red cabbage, red campion, red carpet, red cedar, red cent, red china, red clay, red clintonia, red cloud, red clover, red cole, red complexioned, red coral, red cross, red currant, red cypress pine, red dagga, red deer, red delicious, red devil, red dog, red dogwood, red drum, red dwarf, red dwarf star, red eft, red elm, red faced, red false mallow, red fire, red fox, red giant, red giant star, red ginger, red goatfish, red goosefoot, red gram, red grouse, red guard, red gum, red hand defenders, red handed, red haw, red heat, red helleborine, red herring, red hot, red indian, red ink, red jungle fowl, red juniper, red kauri, red lauan, red lauan tree, red laver, red lead, red light, red light district, red line, red maids, red man, red maple, red marrow, red meat, red morning-glory, red mulberry, red mullet, red notice, red oak, red onion, red osier, red osier dogwood, red panda, red pepper, red periwinkle, red phalarope, red pimpernel, red pine, red planet, red poll, red porgy, red raspberry, red rat snake, red region, red river, red rockfish, red salmon, red sandalwood, red sanders, red sanderswood, red saunders, red scare, red sea, red setter, red shift, red shrubby penstemon, red silk cotton, red silk-cotton tree, red silver fir, red siskin, red snapper, red sorrel, red spider, red spider mite, red sprites, red spruce, red squirrel, red sun, red tai, red tape, red tide, red trillium, red underwing, red valerian, red water, red willow, red wine, red wolf, red worm, rhode island red, river red gum, see red, seeing red, shumard red oak, signal flag, soft flag, southern blue flag, southern red cedar, southern red oak, swamp red oak, sweet flag, turkey red, turn red, union flag, venetian red, water flag, western red cedar, wheat flag smut, white flag, wild red oat, yellow flag, yellow water flag