A rewrite engine is a software component that performs rewriting on Uniform Resource Locators, modifying their appearance. This modification is called URL rewriting. It is a way of implementing URL mapping or routing within a web application. The engine is typically a component of a web server or web application framework. Rewritten URLs (sometimes known as short, pretty or fancy URLs, search engine friendly - SEF URLs, or slugs) are used to provide shorter and more relevant-looking links to web pages. The technique adds a layer of abstraction between the files used to generate a web page and the URL that is presented to the outside world.