stoplight

Noun

 * 1)  A traffic control signal, traditionally consisting of three lights, colored green, yellow/amber, and red, meaning proceed, prepare to stop, and stop, respectively.
 * 2) A light on the  of a  that is activated when ; a.

Usage notes
The two-word form stop light was initially more common, but its usage plateaued around 1950, whereas usage of stoplight increased continually since that year, overtaking stop light around 1970.