Definition from Wiktionary, the free dictionary
stoplight (plural stoplights)
- (US) A traffic control signal, traditionally consisting of three lights, colored green, yellow/amber, and red, meaning proceed, prepare to stop, and stop, respectively.
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.
(US) a traffic control signal