1. noun A red traffic light, which is a signal to 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 By extension, 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 Indicative of a brothel (due to the practice of using red lamps to show that an establishment is a brothel). Often hyphenated. Be careful if you venture into the red-light district—there are a lot of shady characters there.