A functional requirement is a requirement that specifies what the system should do or how it should behave under certain conditions. A functional requirement describes the functionality, behavior, or performance of the system. An example of a functional requirement is the system is able to require passwords, which defines a security feature of the system that prevents unauthorized access. References: PMI Professional in Business Analysis (PMI-PBA)Examination Content Outline1, page 17; Business Analysis for Practitioners: A Practice Guide2, page 92.