Use case

Use case is a description of the observable interaction between an actor (or actors) and a solution that occurs when the actor uses the system to accomplish a specific goal.

Definitions

According to the BABOK Guide (3rd edition),

Use case. A description of the observable interaction between an actor (or actors) and a solution that occurs when the actor uses the system to accomplish a specific goal.

According to the ITIL Foundation 4e by Axelos,

Use case. A technique using realistic practical scenarios to define functional requirements and to design tests.