Which of the following best describes an actor in a use-case diagram?

An actor in a use-case diagram is best described as a role that can be a user or another system interacting with the system being modeled. This characterization emphasizes that actors represent entities that have a goal when they interact with the system, which can include both human users and external systems. By defining actors in this way, use-case diagrams can effectively depict the various interactions that occur within the system and help stakeholders understand who will use the system and for what purpose.

The focus on roles rather than specific users or components allows for a broader interpretation, making it easier to identify all possible interactions. This definition encompasses various forms of interaction that actors can have with the system, whether they are initiating actions, receiving information, or providing inputs to the system. Such clarity is essential in developing a comprehensive understanding of user requirements and system functionalities.