UX Glossary · U
Usability
How effectively, efficiently and satisfactorily specified users achieve specified goals in a specified context.
Usability has a formal definition. ISO 9241-11 describes it as the extent to which a system can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use.
The three qualifiers matter as much as the three measures. Usability is not a property of a product in the abstract. The same interface can be highly usable for a trained operator and unusable for an occasional visitor.
- Effectiveness: can they complete the task, and correctly
- Efficiency: at what cost in time and effort
- Satisfaction: how acceptable was the experience
Nielsen’s components
A commonly used complementary breakdown adds learnability, memorability and error rate to efficiency and satisfaction. These are measurable, which is what separates a usability claim from an aesthetic one.
Usability and user experience
Usability is a component of user experience, not a synonym. A product can be entirely usable and still be unpleasant, distrusted or pointless. UX covers the whole response including anticipation and aftermath; usability covers whether the task can be done.
In practice
When someone says a design is “not usable”, the useful reply is: for which users, doing what, in what context, and measured how. The definition turns an opinion into a testable question.