Usability
Usability describes how easily specific people achieve goals with effectiveness, efficiency and satisfaction in a context of use
What it means, how it is used and where it comes up, with its related concepts.
What it is and how it works
It is evaluated with defined tasks and users. A solution can be usable in one environment and problematic in another. It is better to observe results, errors, effort and feedback than to reduce it to an aesthetic judgement.