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):

traffic signal
    n 1: a visual signal to control the flow of traffic at
         intersections [syn: traffic light, traffic signal,
         stoplight]