What best defines a non-functional requirement?

A non-functional requirement is essentially a specification that describes how a system should perform rather than what it should do. This includes aspects like performance metrics, reliability, security, and usability. It defines the quality attributes of the system that are critical for its operational effectiveness and user satisfaction. For instance, a non-functional requirement might state that a system should handle a certain number of transactions per second or be accessible 99.9% of the time. This helps stakeholders understand the expectations for the system's operation and ensures that the resulting product aligns with users' needs and business goals.

The other choices, while related to system design and requirements, do not encapsulate the essence of non-functional requirements as clearly. A detailed system architecture overview pertains more to the structural aspects of a system, business requirements for stakeholder engagement focus on what the business needs to engage its stakeholders, and user interface design guidelines concentrate on the aesthetics and usability of the interface rather than its overall performance characteristics. Each of these has its own role in system development but does not define non-functional requirements as accurately as specifying performance metrics and operation criteria does.