If a place is overcrowded, there are too many people or too many things in it — more than it can hold in a comfortable or safe way. A crowded train is full. An overcrowded train is so full that people cannot move, or the space feels unsafe. We use this about rooms, buses, cities, classrooms, hospitals, animal cages, and also about a cupboard stuffed with boxes. It is a bit stronger and a bit more serious than just crowded. You hear it in everyday talk and in the news. Though actually, there is no magic number that turns crowded into overcrowded: it is how it feels, and sometimes whether a safety limit has been crossed. It is an adjective: an overcrowded room, overcrowded cities.