On Non-Functional Requirements

Although the term 'non-functional requirement' has been in use for more than 20 years, there is still no consensus in the requirements engineering community what non-functional requirements are and how we should elicit, document, and validate them. On the other hand, there is a unanimous consensus that non-functional requirements are important and can be critical for the success of a project. This paper surveys the existing definitions of the term, highlights and discusses the problems with the current definitions, and contributes concepts for overcoming these problems.