Quality Attribute Scenario
also called QAS
A structured, testable statement of a non-functional requirement: source, stimulus, environment, artefact, response, response measure.
The technique that turns "the system must be scalable" into something you can design against and verify.
Source: a burst of users. Stimulus: 10× normal request rate. Environment: normal operation, peak season. Artefact: the checkout API. Response: requests continue to be served, excess is shed cleanly. Response measure: p99 stays under 800 ms, error rate under 1%, no manual intervention.
That is now a design constraint, a load-test specification and an acceptance criterion in one sentence. Six or eight of these scenarios usually capture everything architecturally significant about a system's quality requirements — which is a far more tractable artefact than a hundred-page NFR appendix nobody reads.