A Composition is used to depict an element that is made up of smaller components, typically in a Class or Package diagram. A component - or part instance - can be included in a maximum of one composition at a time. If a composition is deleted, usually all of its parts are deleted with it; however, a part can be individually removed from a composition without having to delete the entire composition. Compositions are transitive, asymmetric relationships and can be recursive.
OMG UML Specification:
The OMG Unified Modeling Language specification, (v2.5.1, p.112) states:
Composite aggregation is a strong form of aggregation that requires a part object be included in at most one composite object at a time. If a composite object is deleted, all of its part instances that are objects are deleted with it.
Compositions may be linked in a directed acyclic graph with transitive deletion characteristics; that is, deleting an object in one part of the graph will also result in the deletion of all objects of the subgraph below that object. The precise lifecycle semantics of composite aggregation is intentionally not specified. The order and way in which composed objects are created is intentionally not defined. The semantics of composite aggregation when the container or part is typed by a DataType are intentionally not specified.