Definitions in semantic are groups of css, fonts, images, and javascript which make up a single element. Unlike other javascript libraries, semantic UI elements are stand-alone and only require their own assets to function correctly.
-CSS Definitions avoid conflicts with other elements by descending rules from a common ui namespace. Any element on a page that has the class ui matches up to a UI definition. Elements without the class ui are a part of a UI definition. The UI namespace allows for individual class names to not alter other element's design, as they are defined only to modify specific nouns.
UI namespaces should use a single word class name that represents its function descriptively on a web site. For example, a button will be namespaced in html using the class name "ui button".
-Semantic has five different ui definitions. These are useful patterns for describing re-usable parts of a website.