A lamp on top of a tall pole beside a road, which lights the street at night. Cities have rows of them; they usually switch on by themselves at dusk and off at dawn. The word covers both the lamp itself and the whole pole-and-lamp thing. You will also see "street light" as two words and "street lamp" or "streetlamp" — all common, all correct, and you can treat them as the same word. "Streetlight" as one word is the most usual spelling in American English. Beyond the plain meaning, streetlights show up constantly in stories and songs as a picture of a lonely city night — standing under a streetlight waiting for someone, the orange glow on wet roads. There is also a well-known joke that has become a way of talking about lazy research, "looking under the streetlight": a man searches for his lost keys under the lamp, not because he dropped them there, but because that is where the light is.