To overcrowd means to fill a space with too many people or things, making it uncomfortable, unsafe, or unable to function properly. When a classroom is overcrowded, there might not be enough desks for everyone, or students might have trouble seeing the board. When a bus is overcrowded, passengers squeeze together with barely room to breathe.

The word suggests exceeding capacity in a way that causes problems. A movie theater can be completely full without being overcrowded if every seat is occupied and everyone's comfortable. But if 200 people cram into a theater designed for 150, it becomes overcrowded: aisles get blocked, people can't see, and the space stops working the way it should.

Overcrowding creates real problems. Overcrowded animal shelters struggle to care for all their animals properly. Overcrowded cities face challenges with housing, transportation, and resources. An overcrowded schedule leaves no time to rest or think clearly.

You can also use overcrowded as an adjective to describe a space or situation: “The playground was overcrowded during recess, so we could barely play kickball.” Notice that something becomes overcrowded when it passes its reasonable capacity, not just when it feels busy or full.