Non-Functional Requirement
also called NFR, Quality Attribute
A requirement about how well the system must behave rather than what it must do — latency, availability, throughput, security, cost.
Functional requirements describe behaviour: the system must let a user reset their password. Non-functional requirements describe the quality of that behaviour: the reset email must arrive within 30 seconds for the 99th percentile, the endpoint must survive 500 requests per second, and the reset token must be unguessable.
NFRs are where architecture actually gets decided. Almost any structure can satisfy the functional requirements; only a few can satisfy the functional requirements and hit 99.95% availability and stay under the cost ceiling. That is why an NFR without a number is not a requirement — "the system must be fast" constrains nothing and can never be tested.
A usable NFR names the metric, the target, the percentile and the condition: "p99 API latency under 300 ms at 1,000 RPS sustained, measured at the gateway."