Aggregation is a type of class or object association.  Aggregations indicate a whole-part relationship, and are known as "is part of" or "has-a" relationships. An Aggregation relationship is indicated by a line with a hollow diamond.

UML Symbol:

Aggregation vs. Composition

It's interesting to contrast aggregation with composition, which is also known as composite aggregation. General aggregation does not indicate whether the class can exist by itself or not. It simply designates a whole-part relationship. Composition indicates the class cannot exist by itself such as a button class control must exist as part of a container form class.

Summary of Relationship types

An aggregation relationship represents an "is-part-of" or "has-a" relationship. Contrast this with composition which represents a "uses-a" relationship. You can also contrast this with inheritance which represents an "is-a" relationship and with interfaces which represents a "behaves-as" or "looks-like" relationship.