Wikipedia says:
Usability is a term used to denote the ease with which people can employ a particular tool or other human-made object in order to achieve a particular goal. Usability can also refer to the methods of measuring usability and the study of the principles behind an object’s perceived efficiency or elegance.
In human-computer interaction and computer science, usability usually refers to the elegance and clarity with which the interaction with a computer program or a web site is designed. The term is also used often in the context of products like consumer electronics, or in the areas of communication, and knowledge transfer objects (such as a cookbook, a document or online help). It can also refer to the efficient design of mechanical objects such as a door handle or a hammer.
Google starts with:
Quite simply, usability is making your site easy for your customers to find the exact information they need when they need it. …
www.searchenginewriting.com/glossary.shtmlA set of attributes that bear on the effort needed for use, and on the individual assessment of such use, by a stated or implied set of users. Learnability Understandability Operability
www.bigpedia.com/encyclopedia/ISO_9126the ease of use, learnability, efficiency, and error tolerability of a particular product.
www.utexas.edu/academic/diia/assessment/iar/glossary.php
I still haven’t decided about my own definition. It might take a while, it’s not an easy task.
