Wikitext is a document written in a wiki markup language, used on Mediawiki sites such as wikipedia.org.
From MediaWiki.org:
Wikitext is a document written in a wiki markup language, such as the current one explained in Help:Editing (see also Markup spec/DTD). It is a mixture of content, markup, and metadata. The current and old versions of all pages of a wiki are stored in the database in the text table, in the form of wikitext.