Definition of use case:
(n) :
(software engineering) A usage scenario for a piece of software; often used in the plural to suggest situations where a piece of software may be useful.
(n) :
(software engineering) A potential scenario in which a system receives an external request (such as user input) and responds to it.
(n) :
(engineering, generally) A usage scenario for a product, such as a piece of equipment or a tool.