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Nonfunctional Requirements
A non-functional requirement (NFR) is a requirement that specifies criteria that can be used to judge the operation of a system, rather than specific behaviors (a functional requirement). Non-functional requirements are often called "quality attributes", "constraints" or "non-behavioral requirements".
In plain English, non-functional requirements are things that 100% is needed for the code to run. For example, a requirement would be to be able to login, but whether it takes 1 second to log in or 1 hour to login, both satisfy the functional requirement. The non-functional requirement would then be that the login time should take no more than 3 seconds.
Informally, these are sometimes called the "ilities", from attributes like stability and portability. NFRs can be divided into two main categories:
- Execution qualities, such as safety, security and usability, which are observable during operation (at run time).
- Evolution qualities, such as testability, maintainability, extensibility and scalability, which are embodied in the static structure of the system
Usually you can refine a non-functional requirement into a set of functional requirements as a way of detailing and allowing (partial) testing and validation.
Examples:
- The printer should print 5 seconds after the button is pressed
- The code should be written in Java
- The UI should be easily navigable
More Information:
- Wikipedia article
- ReQtest Explains the difference between functional and nonfunctional requirements
- Scaled Agile Works through the process from finding to testing nonfunctional requirements