traffic light

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A road signal for directing vehicular traffic by means of colored lights, typically red for stop, green for go, and yellow for proceed with caution.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A signalling device positioned at a road intersection or pedestrian crossing to indicate when it may be safe to drive, ride or walk, using a universal colour code.
• noun philately The coloured dots on stamp sheet margins printed with offset litho or photogravure methods, used by the printers to check colour accuracy.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a visual signal to control the flow of traffic at intersections