Glossary Usability
Usability
DesignTech
Definition
How well a specific person can use a thing to reach a specific goal — effectively, efficiently, and without regret — measured by watching them try
ISO 9241's three words: effectiveness, efficiency, satisfaction. Usability is a property of a person plus a task plus an interface, never of the interface alone, which is why it is tested rather than reviewed.
Whether it works for the person in front of it. Accessibility asks whether it works for every person; usability asks whether it works well for this one. A usable thing can be inaccessible; an accessible thing can be unusable.