Use Case

A collection of possible scenarios between the system under discussion and external actors, characterized by the goal the primary actor has toward the system's declared responsibilities, showing how the primary actor's goal might be delivered or might fail.
Use cases are goals (use cases and goals are used interchangeably) that are made up of scenarios. Scenarios consist of a sequence of steps to achieve the goal; each step in a scenario is a sub (or mini) goal of the use case. As such, each sub goal represents either another use case (subordinate use case) or an autonomous action that is at the lowest level desired by our use case decomposition.