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Aggregation in Object-Oriented Programming (OOP) is a specific kind of "has-a" relationship between objects, where one object is a part of another object, but both can exist independently. It's a way to represent a whole-part relationship.

Let's use a simple real-world analogy to explain this concept. Think of a classroom setting. A classroom "aggregates" students. Each student is a separate, independent entity, but when they come together in a classroom, they form a collective group - the class. However, the students are not bound to the classroom; they can leave the class, join another class, or exist without being in any class at all.

In programming, this is similar. For instance, consider a Car class and a Wheel class. A car needs wheels to function properly, so in this context, the Car class aggregates Wheel objects. However, the existence of a Wheel is not dependent on a Car. Wheels can be created and destroyed independently of a car.

Key characteristics of aggregation:

1. Individuality: The parts (like students or wheels) can exist independently of the whole (like a classroom or car).

2. Loose Coupling: The whole and parts are loosely coupled. This means changes to the part class do not severely affect the whole class, and vice versa.

3. Lifespan: The lifecycle of the parts is not managed by the whole. For example, destroying a Car object doesn't necessarily destroy its Wheel objects.

Aggregation is useful for representing relationships where components are part of a larger complex object but also maintain their independence. This concept helps in designing systems that are modular, easier to manage, and maintain.