The term "audience" is often used by technical writers as well as web designers. If you understand the characteristics of the people who are most likely to use a website, you can design the website to fit their needs.
For example, while a cutting-edge website for people in their 20's can use tiny characters, a site designed for seniors should always use large, easily readable characters.
A usable website is one that has been designed with a clear idea of who the audience is (or, more likely, who the audiences are).
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