Screenshot of the edit window in a Wikipedia article. Note the <nowiki> tag, used to escape wiki markup and HTML. HTML comments can be seen inside the <!-- --> tags.
Wiki markup, also known as wikitext language and wikicode, is a lightweight markup language used to write pages at wiki-based websites (such as Wikipedia) that is a simplified alternative/intermediate to HTML. Its ultimate purpose is to be converted by wiki software into HTML, which, in turn, is served to web browsers. It was originally created in 1995 to format pages on the original wiki site, WikiWikiWeb.
There is no commonly accepted standard wikitext language. The grammar, structure, justification, keywords and so on depend on the particular wiki software used on the particular website. For example, all wikitext markup languages have a simple way of hyperlinking to other pages within the site, but there are several different syntax conventions for these links. Many wikis, especially the earlier ones, used CamelCase to mark words that should be automatically linked. In MediaWiki, this convention was replaced with the [[…]] notation, which Wikipedia calls "free links".
Different Wiki programs may support use of different sets of HTML elements within wikitext. In some cases, permitted HTML elements may be configured by individual wiki sites. MediaWiki supports many common HTML tags.
Creole is an effort for a "common wiki markup language to be used across different Wikis". There are several wiki engines that have implemented Creole. Version 1.0 of the specification was released in July 2007. It is not supported by MediaWiki.