use case definition

I propose the following definition for use case.  I've also included some 
additional possibilities. It may make sense to use some of this (#2-4) in 
the Notes below the checkpoint.  #5 is a definition for User Scenario

1. USE CASES help to identify and clarify the functional requirements of 
the specification.  Use cases capture all the different ways a 
specification would be used, including the set of interactions between the 
user and the specification as well as the services, tasks, and functions 
the specification is required to perform.

However, here is some Background or alternative definition

2. A use case defines a goal-oriented set of interactions between external 
actors and the system (specification) under consideration. (Actors are 
parties outside the system that interact with the system (UML 1999, 
pp2.113-22.123)
An actor may be a class of users, roles users can play or other systems.

3. Use cases capture (who(actor) does what (interaction) with the system, 
for what purpose (goal) without dealing with system internals.  A complete 
set of use cases specifies all the different ways to use the system, and 
therefore defines all behavior required of the system, bounding the scope 
of the system.

4.  Use case steps are written in an easy-to-understand structured 
narrative using the vocabulary of the domain.

5.  USER SCENARIO - an instance of a use case, representing a single path 
through the use case.  Thus, there may be a scenario for the main flow 
through the use case and another scenarios for each possible variation of 
flow through the use case (e.g., representing each option).


regards
lynne