A wiki is a type of website that allows users to collaboratively edit its content and structure from within a web browser. Most wiki sites are a collection of hyperlinked pages that serve as a knowledge base for an organization or online community.
Wiki software is a style of content management system (CMS) running on a web server, also called a "wiki engine." Many different wiki engines are available. Some wiki engines are designed for public online communities, while others specialize in internal enterprise use. Public wikis usually emphasize large user bases and community discussion, allowing anyone to sign up and edit pages. Internal enterprise wiki sites instead focus on controlling access and editing privileges, integrating with other software, and managing a document library.
A wiki administrator can choose whether to allow any user to edit content or restrict privileges to registered users — they can even specify multiple classes of users with different roles and permissions. Any user that can edit a wiki can do so using a simple markup language (or sometimes a rich text editor) directly in a web browser window without any extra software. Since allowing users to edit a wiki page's content can lead to accidental loss or vandalism, a wiki engine will keep track of every page's edit history so that other users can roll back unwanted changes. A wiki engine can also require approval of certain changes before they are published.
NOTE: The most prominent example of a wiki on the Internet is Wikipedia.