Usability is the extent to which specific users achieve specific goals in specific contexts

What it is

The standard definition of usability (ISO 9241-11) is a product of three qualifiers—specific users, specific context, specific goals. It measures effectiveness, efficiency, and satisfaction for that combination, not an abstract property of the product. Saying "this product is highly usable" without naming the users or the task says nothing.

Why it happens

Usability is a relational property, not an object property: it lives in the match between user capability, task demands, and product features. The same product is an efficient tool for experts and a maze for novices because the match changed. Each qualifier imports variables—user knowledge and ability, physical and social context, goal structure and tolerance for error—and changing any one can flip the verdict.

Studying it

Fix the three elements first in any measurement design: recruit a sample of the target users, run the goal task in a representative context (lab setting or field), and record effectiveness (completion, errors), efficiency (time, steps), and satisfaction (scales). Changing any element requires re-measurement, and comparing usability numbers across studies requires checking that the three elements match; otherwise the figures are incomparable.

Where it stops holding

The definition covers "can it be used, is it good to use," not motivation, aesthetics, or meaning; it explains frequent, task-oriented use better than infrequent exploratory use. "Context" can only be approximated in the lab, and missing field factors systematically shift results. Satisfaction, being subjective, is shaped by expectation and culture and should not be compared directly across groups.

Applying it

• In requirements, rewrite "easy to use" into the three-element sentence: for which users, in which context, achieving which goal.
• Open test reports with the three-element setup, and scope conclusions to that setup.
• Before citing usability data in decisions, check user group and task alignment; when they differ, draw no conclusion.

Related

• Same group: B5.01.2 Usability is meaningless to discuss apart from users, goals, and context · B5.01.3 Usability is a measurable property, not an evaluative adjective
• Nearby: B5.02 Effectiveness, efficiency, and satisfaction · B5.05 Experience versus usability
• Search terms: usability definition · iso 9241-11 · context of use