Aggregation is a special form of association that represents a has-a relationship between two classes¹. Aggregation is also known as a whole-part relationship, because one class (the whole) has a reference to another class (the part) as an attribute².

Aggregation is different from composition, which is a stronger form of whole-part relationship, where the part cannot exist without the whole. In aggregation, the part can exist independently of the whole, and the whole is not responsible for the creation or destruction of the part².

Aggregation is also different from a simple association, which is a more general relationship between two classes that does not imply ownership or containment. In aggregation, the whole has a more specific role and purpose than the part, and the part contributes to the functionality or characteristics of the whole³.

Here is an example of aggregation between classes in Java:

In this example, the Institute class has an aggregation relationship with the Department class, because an institute has one or more departments as its parts. Similarly, the Department class has an aggregation relationship with the Student class, because a department has one or more students as its parts. However, both departments and students can exist independently of their respective wholes, and they are not created or destroyed by them. Therefore, aggregation is a suitable way to model these relationships.