Capturing the dynamic behavior of a system.(dynamic behavior refers to the behavior of the system when it is running/operating.
use case diagram is dynamic in nature, the internal and external factors for making interaction

Use Case Diagram consists of actor, use cases and their relationship

A single use case diagram captures a particular functionality(特定功能) of a system!

Purpose of Use Case Diagrams

Use Case Diagrams are to capture the dynamic aspect of a system. ====》 the definition is too generic to describe the purpose.

Use Case Diagrams are to gather the requirements of a system

When a system is to gather its functionalities, use cases are prepared and actors are identified.