UseCase

Here's a more technical definition:

"The essence of the use case model is to capture and/or structure user requirements of a new system, whether it is being developed from scratch or based on an existing system, by identifying all the big functional blocks (use cases) and by detailing all the scenario’s that users will be performing (within these big functional blocks). The use case model helps the customer, users and developers agree on how to use the system." -- What are use cases?

See:

• Use Case Fundamentals - An introductory paper by Alistair Cockburn.
• What’s the Problem? - Another good introduction to UseCases from A List Apart.
• Use Case Zone - "A site dedicated to providing links and information on use cases."
• The Value of Use Case Methodology - Good introductory paper with a use case template.
• An Introduction to Use Case Analysis - A more technical article.