A UML term used to describe a form of association in which the lifetime of the whole controls the lifetime of the parts. In a composition, the instances of two classes depend on each other. The whole controls the location and lifetime of its parts. For example, in ArcMap, a map is composed of layers. If you move a map on a layout, the layers move, and if you delete the map, its layers get deleted; therefore, the lifetime of these objects depend on one another.