Cascading Style Sheet (CSS)
From Joomla! Documentation
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A Cascading Style Sheet or CSS is used to control the presentation of an HTML page. For example, a CSS file will often control the font, margins, color, background graphics, and other aspects of a web page's appearance. CSS allows you to separate the content of an HTML page from it's appearance. In Joomla!, CSS files (for example, template.css) are normally part of the template.
See also: Template, Page Class Suffix, Module Class Suffix