Match each of the key terms with the definition that best fits it.

____________ A class that has no direct instances but whose descendants may have direct instances.

Here are the key terms from the appendix. The page where each term is first explained is in parentheses after the term.

1. Abstract class

2. Activation

3. Actor

4. Aggregation

5. Association

6. Association role

7. Behavior

8. Class diagram

9. Component diagram

10. Concrete class

11. Encapsulation

12. Event

13. Multiplicity

14. Object

15. Object class

16. Object diagram

17. Operation

18. Sequence diagram

19. Simple message

20. State

21. State transition

22. Synchronous message

23. Unified Modeling Language (UML)

24. Use case

25. Use-case diagram