Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design and the Unified Process, Second Edition

6.3. Use Cases and Adding Value

First, some informal definitions: an actor is something with behavior, such as a person (identified by role), computer system, or organization; for example, a cashier.

A scenario is a specific sequence of actions and interactions between actors and the system under discussion; it is also called a use case instance. It is one particular story of using a system, or one path through the use case; for example, the scenario of successfully purchasing items with cash, or the scenario of failing to purchase items because of a credit card transaction denial.

Informally then, a use case is a collection of related success and failure scenarios that describe actors using a system to support a goal. For example, here is a ...