In the context of requirements, what defines a 'use case'?

A use case is fundamentally defined as a description of how users interact with a system. It captures specific scenarios in which a user will interact with the system to achieve a particular goal. Each use case outlines the steps that the user would take, the various conditions under which those steps occur, and the expected outcomes.

By focusing on user interactions, use cases help to clarify requirements from the perspective of the end users. This approach not only aids in understanding how the system should function but also ensures that the developers and stakeholders are aligned on user needs and experiences.

In contrast, the other options articulate different aspects of system development. They may focus on system capabilities, performance requirements, or project constraints, but they do not emphasize the user's interaction with the system as use cases do. Therefore, the defining characteristic of a use case lies in its focus on the user's experience and interaction with the system.