A wiki (sometimes wikiwiki) is a website that allows anyone (with permission required or not, depending on the wiki) to edit any page and to add new pages. Wikis are unusually collaborative because all participants can, in the typical case, edit all, or nearly all, pages on a wiki. This is true of the best-known wiki, Wikipedia.
"Wiki" is the Hawaiian word meaning "quick," and as Hawaiian reduplicates words to modify them, "wikiwiki" means "very quick." Ward Cunningham, during his first visit to the Islands, was directed to a "Wiki Wiki" shuttle between airport terminals. He says, "Using the word-doubling convention, I named the technology WikiWikiWeb." 
Wiki software is available in several variants, including MediaWiki and TWiki. It is now being widely used in business and educational institutions for both intranet and internet applications, where it is recognized as a significant productivity tool.
What is special about wikis?
Wikis have several characteristics that make them special compared to other kinds of websites.
- Easily editable. Just a click on a button or a link is enough to allow a user to change the content of a wiki. This reduces drastically the distance between viewing a site and contributing to it.
- Historization. Most wikis have extensive page histories. All changes to a wiki page are memorized and kept in archives. One can view and compare the previous versions, reinstate text, merge two versions or simply restore an older one.
- Easy creation of new pages. Wikis make it possible for users to easily create new pages, through a simple syntax.
- No broken links. In a traditional wiki, there cannot be any broken link. If a user requests a page that does not actually exist, a form opens to invite him to bring content and create the page.
- A posteriori moderation. Contrary to many other collaborative sites, wikis rely on the principle that users can create content that is moderated afterwards (if need be): no validation is requested before the content appears online.
- Another way to look at site creation. The classical way of creating a site is to think of its structure first, then design it and bring the content. In a wiki, there is just a basic structure at the beginning, if any. Users then start bringing content and creating new pages and the wiki is being constructed step by step without a preliminary site plan. Gradually, the wiki structure evolves to become more usable, notably through reorganization (or "refactoring") of pages: deletion, merging, division of pages, etc.
Ward Cunningham invented the wiki concept and created the first wiki, WikiWikiWeb, on March 25, 1995. WikiWikiWeb, also called "Ward's Wiki," began life as a supplement to the Portland Pattern Repository.
Until 2001 or 2002, perhaps most wikis were general-purpose communities, with only very vague missions and an idiosyncratic set of rules and processes, in which they tended to emulate WikiWikiWeb.
Thus, shortly after Wikipedia got its start in 2001, when Cunningham was asked whether he thought wiki software could be used to create an encyclopedia, he replied, "Yes, but in the end it wouldn't be an encyclopedia. It would be a wiki." Whether Cunningham was correct is a matter of opinion. But regardless, Wikipedia did develop a different set of rules and processes, which spawned another set of wiki clones.
Issues and concerns about wikis
For all their benefits, wikis still raise some issues or at least concerns that are still debated among wiki users.
- Vandalism. The most open wikis allow anyone to edit their pages, without even requesting to log in. In that case, changes that are made are quite anonymous: the only trace left is an IP address, which does not make it possible to identify the author of a change. This is still the way Wikipedia operates today, even though changes have been made in its policies to request log-in, in some cases. Anonymity can indeed be problematic in case of misbehavior. The danger here is often described as vandalism: an anomymous user can erase or alter the content of a page. Historization is a way to counter-balance this risk, since it is always possible to restore a previous version.
- Trustworthiness. The fact that edition can be anonymous, or pseudonymous, raises the question of the trustfulness of information. To solve this issue, some wikis (Citizendium is an example) request users to create an account and log in before they can edit pages. The use and verification of real names is an answer to the trustworthiness issue.
- Ergonomics. Wiki software lacks uniformity in its syntax. Wiki engines have different ways of emphasizing texts (bolding and italicising), inserting images and creating links. Wiki syntaxes require some training from users and WYSIWYG is not the common case. This makes it difficult for beginners to edit wikis and partly explains the gap between the numbers of wiki viewers and wiki editors.
- Best and worst practices. Because wikis have been around for a number of years, examples of best and worst practices are becoming known. These are especially useful if a wiki is being used within an organization, as a vehicle for knowledge management.