ISO 9241-11:2018 includes the following:-
Usability is a more comprehensive concept than is commonly understood by “ease-of-use” or “user friendliness”.
Usability is relevant to:
- regular ongoing use, to enable users to achieve their goals effectively, efficiently and with satisfaction;
- learning, to enable new users to become effective, efficient and satisfied when starting to use a system, product or service;
- infrequent use, to enable users to be effective, efficient and satisfied, with the system on each reuse;
- use by people with the widest range of capabilities;
- minimizing the risk and the undesirable consequences of use errors; and
- maintenance, in that it enables maintenance tasks to be completed effectively, efficiently and with satisfaction.
See also User Experience Basics from usability.gov
Usability Engineering means designing a website to be user centred (rather than technology centred), and involving users in the design process.
Usability Testing means measuring the usability of a website (or product).
Usability Consultant is someone who assesses the usability of a website.