A requirement is an externally observable characteristic of a desired system. When trying to determine whether a candidate requirement is really a requirement, two criteria from this definition must be met: (1) it must be externally observable, and (2) it must be desired. Externally observable implies that there exists some means by which an observer (whether this is a human, machine, animal, or another system) can sense that the system satisfies the requirement from a perspective external to the system itself. Desired implies that some stakeholder (q.v.) wants to have that characteristic present in the system.

In addition to being externally observable and desired, there are numerous other characteristics that a requirement should exhibit and these are discussed.