red light
1. noun On a traffic light, the color that means "stop." I pulled up to the red light and stopped the car. Please be careful driving out there—people are maniacs and don't even stop at red lights!
2. noun A message or signal to stop. I'm really not interested in dating Rob, and I thought I gave him a red light, but he keeps pursuing me nonetheless.
3. adjective (often hyphenated) Indicative of a brothel (due to the practice of using red lamps to show that an establishment is a brothel). Be careful if you venture into the red-light district, there are a lot of shady characters there.