From Jennifer Kyrnin,
Your Guide to Web Design / HTML.
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Definition: The client or application which initiates a request to a Web server or other Internet application. Typically, the user-agent is a general term for a Web browser, but can include screen readers, spiders, Web editors or programs that access Web pages.

Pronunciation: ay jEnt (noun)

Also Known As: agent

Alternate Spellings: user-agent

Examples: Firefox and Internet Explorer are user agents.