A non-functional requirement defines a quality target or constraint that a system must satisfy.
It describes how well the system must operate. For example, it may address performance, security, reliability, usability, scalability, or availability.
A functional requirement defines what the system must do. In contrast, a non-functional requirement defines the required quality or operating condition. For example, a system may provide a search function and display results within two seconds.
Teams should define measurable criteria. Therefore, they should replace vague terms such as fast, secure, or user-friendly with specific targets. These targets may include response times, availability rates, error limits, or user success rates.
Constraints may also limit technologies, platforms, legal conditions, or operating environments. Moreover, these limits often influence architecture and design decisions.
Clear quality targets and constraints support objective testing, informed design decisions, and reliable system acceptance.
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