1A yellow or amber warning light used to instruct traffic; especially the middle of three lights in a set of traffic lights, serving as a warning that the signal is about to change to red (stop) or green (go). Compare "red light", "green light", "yellow".
2Figurative and in figurative contexts. A cautionary indicator or signal; specifically (a) qualified or tacit permission to proceed in a particular course of action; (b) a warning of likely difficulties ahead, requiring heightened caution or vigilance.
1920s; earliest use found in The Kansas City Star. From yellow + light.