Stands for "Cascading Style Sheet."
CSS is a style sheet language used for formatting content in HTML webpages. CSS style sheets can define the appearance and formatting of text, tables, and other elements separately from the content itself. Styles may be found within a webpage's HTML file or in a separate document referenced by multiple webpages.
CSS helps web developers create a uniform look across an entire website. Instead of formatting the appearance of each table and block of text in a webpage's HTML code, a style is defined once in a CSS style sheet. Common HTML formatting tags, like <h2>, <strong>, and <em> can have custom formatting defined in a CSS file; custom styles can also be created and applied to text, images, and tables. Once a style is defined, it can be used by any page that links to the CSS file.
By separating a webpage's content from its formatting, CSS makes it easy to update styles across several pages at once. For example, if you want to increase the body text size from 10pt to 12pt across dozens of separate HTML pages, you only need to change the style once in the CSS file. The text size changes for every instance of that style on any webpage using that style sheet.
NOTE: The word "cascade" refers to the priority scheme used by CSS when multiple style rules overlap. User-defined CSS overrules styles applied directly to an HTML tag, which overrule any styles defined within the HTML document's header, which overrule any styles defined in an external CSS file.