The Collaborative International Dictionary of English v.0.48:

Red light \Red light\
   1. an electrically operated set of lights at a road
      intersection which has different lights visible to traffic
      from different directions, designed to control vehicle
      traffic through the intersection. Each set of lights
      typically has a colored red light and also green and amber
      lights; the color of the light which is lighted at any one
      time changes automatically to control the flow of traffic
      through the intersection, allowing flow from different
      directions in alternating succession. Also called
      traffic light, traffic signal or stop light.
      When the red light is illuminated the signal means to
      stop; green means to go; and amber means to stop or
      procede through the intersection with caution. In simple
      intersections of two roads, a red light visible to traffic
      on one road will usually be accompanied by a green light
      visible to traffic on the intersecting road. In some
      locations the lights may be set to be illuminated in other
      sequences or combinations; a blinking red light is
      typically equivalent to a "stop" sign, and a blinking
      amber light typically means "procede with caution".
      [PJC]

   2. the condition of a traffic light when the signal visible
      to the driver of a vehicle is red, signalling that the
      vehicle must stop and not enter the intersection; I got
      seven red lights on the way to work.
      [PJC]

   3. figuratively, a sign or signal that one must stop doing
      what one is presently doing; as, a fatal side effect is
      often a red light to continuation of a clinical trial.
      [PJC]

WordNet (r) 3.0 (2006):

red light
    n 1: a cautionary sign of danger [syn: red light, warning
         light]
    2: the signal to stop

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "red light":
   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 flag,
   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, stop-and-go 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