Here is the answer.
Aggregation is a relationship between two classes that is best described as a "has-a" and "whole/part" relationship. It is a more specialized version of the association relationship.
The aggregate class contains a reference to another class and is said to have ownership of that class. Each class referenced is considered to be part-of the aggregate class
Aggregation differs from ordinary composition in that it does not imply ownership. In composition, when the owning object is destroyed, so are the contained objects. In aggregation, this is not necessarily true.
For example, a university owns various departments (e.g., chemistry) , and each department has a number of professors. If the university closes, the departments will no longer exist, but the professors in those departments will. Therefore, a University can be seen as a composition of departments, whereas departments have an aggregation of professors.