The ease with which visitors are able to use a Web site.
Web site usability is not just about making sure everything on the site works, but how quickly and easily visitors are able to make use of the site.
The need for Web site usability is perhaps best expressed by Jakob Nielsen’s Law of the Internet User Experience – “Users spend most of their time on other sites.”
Usability is a concern for marketers because of its potential impact, positive or negative, on marketing metrics such as conversion rates, which in turn affect profitability.
see also: Web Design Tools