A Functional Requirement is a specific behavior or function that a system or product must perform. It outlines what the system should do, detailing the expected interactions between the user and the system. For example, in a software application, a functional requirement might state that the user must be able to log in using a username and password. These requirements are essential for guiding the development process and ensuring that the final product meets user needs. They can include features like data processing, user interface elements, and system responses. Properly defined functional requirements help in creating a clear roadmap for developers and stakeholders involved in the project.