attributes of the system not needed to make the system functional but to enhance its service. example: performance, security, quality, maintainability, extensibility
describes a restriction/ constraint on the system that limits our choices for constructing a solution to the problem  includes: safety, security, reliability, portability, usability
- define system properties and constraints e.g. reliability, usability, response time etc. - can be product (e.g. performance), organisational (e.g. authentication) or external (e.g. legal)