An aggregation relationship depicts a classifier as a part of, or as subordinate to, another classifier.
Data flows from the whole classifier (the aggregate) to the part. A part classifier can belong to more than one aggregate classifier and it can exist independently of the aggregate. For example, a Department class can have an aggregation relationship with a Company class, which indicates that the department is part of the company. Aggregation is closely related to composition.
You can name any association to describe the nature of the relationship between two classifiers; however, names are unnecessary if you use association end names.
An aggregation association appears as a solid line with an unfilled diamond at the association end, which is connected to the classifier that represents the aggregate.