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The symbol that represents aggregation in UML (Unified Modeling Language) is an unfilled diamond. This symbol is used to indicate a whole-part relationship between classes, where one class (the whole) contains one or more instances of another class (the part). This relationship implies a "has-a" relationship without the strong ownership that composition suggests.
In UML diagrams, an unfilled diamond is placed at the end of the association line nearest to the class that represents the whole. This represents that while the whole can exist independently of the part, the part can still exist independently of the whole, demonstrating a weaker relationship than composition, which uses a filled diamond to indicate stronger ownership and dependency.
Understanding this distinction is crucial when modeling relationships in object-oriented design, as it helps clarify how different parts of your system interact and share responsibility.