What defines a 'use case' in Agile software development?

In Agile software development, a 'use case' specifically defines a requirement for user interactions. It describes a scenario in which a user (or another system) interacts with the software to achieve a specific goal. Use cases are important because they help capture functional requirements from the end-user’s perspective, ensuring that the development team understands what users need from the system. This focus on user interactions allows teams to prioritize features that deliver real value to the end-users within the Agile framework.

This approach contrasts with other potential definitions. While validating software and detailing functionality are important practices in software development, they don't capture the essence of user interaction and engagement provided by use cases. Testing based on internal structures relates more to white-box testing techniques that examine how software performs, rather than how users interact with it.