A sleeper is something or someone that turns out to be surprisingly successful, powerful, or important after initially seeming unremarkable or being overlooked.

In sports, a sleeper is a team or athlete nobody expects to win. At the start of a basketball season, one team might look weak on paper, but if they make it to the championship, everyone calls them a sleeper. Fantasy sports players look for sleeper picks: athletes who aren't famous yet but might perform brilliantly.

In entertainment, a sleeper hit is a movie, book, or song that becomes wildly popular even though it had a quiet release with little advertising. A small film might open in just a few theaters, then spread through word of mouth until it's playing everywhere and earning millions. The opposite would be a highly advertised blockbuster that everyone expects to succeed.

A sleeper surprises people by looking ordinary on the outside while concealing something special underneath. When you're picking teams for kickball and someone chooses the quiet kid who turns out to be incredibly fast, that's discovering a sleeper.

The word also simply means a person or animal that sleeps, or a type of railroad car with beds for overnight trips.