A wiki is a form of hypertext publication on the internet which is collaboratively edited and managed by its audience directly through a web browser. A typical wiki contains multiple pages that can either be edited by the public or limited to use within an organization for maintaining its internal knowledge base. Its name derives from the first user-editable website called WikiWikiWeb – wiki is a Hawaiian word meaning 'quick'.

Wikis are powered by wiki software, also known as wiki engines. Being a form of content management system, these differ from other web-based systems such as blog software in that the content is created without any defined owner or leader. Wikis have little inherent structure, allowing one to emerge according to the needs of the users. Wiki software usually allows content to be written using a lightweight markup language and sometimes edited with the help of a rich-text editor. There are dozens of different wiki software packages in use, both standalone and part of other software. Some are free and open-source, whereas others are proprietary.

There are hundreds of thousands of wikis in use, both public and private, including wikis functioning as knowledge management resources, note-taking tools, community websites, and intranets. Ward Cunningham, the developer of the first wiki software, WikiWikiWeb, originally described wiki as "the simplest online database that could possibly work".

The online encyclopedia project Wikipedia is the most popular wiki-based website, as well as one of the internet's most popular websites. Wikipedia is not a single wiki but rather a collection of hundreds of wikis, with each one pertaining to a specific language, making it the largest reference work of all time.

In their 2001 book The Wiki Way: Quick Collaboration on the Web, Ward Cunningham and co-author Bo Leuf described the essence of the wiki concept:

• "A wiki invites all users—not just experts—to edit any page or to create new pages within the wiki website, using only a standard web browser without any extra add-ons."
• "Wiki promotes meaningful topic associations between different pages by making page link creation intuitively easy and showing whether an intended target page exists or not."
• "A wiki is not a carefully crafted site created by experts and professional writers and designed for casual visitors. Instead, it seeks to involve the typical visitor/user in an ongoing process of creation and collaboration that constantly changes the website landscape."

Some wikis present users with an edit button or link directly on the page being viewed. This opens an interface for writing, formatting, and structuring page content. The interface may be a source editor, which is text-based and employs a lightweight markup language (also known as wikitext, wiki markup, or wikicode), or a visual editor. The syntax and features of wiki markup languages can vary greatly among implementations.

While wiki software has traditionally offered source editing to users, in recent years some implementations have added a rich text editing mode. This is usually implemented using JavaScript as an interface which translates formatting instructions chosen from a toolbar into the corresponding wiki markup or HTML. An example of such an interface is the VisualEditor in MediaWiki, the wiki software used by Wikipedia.

Some wiki implementations keep a record of changes made to wiki pages, and may store every version of the page permanently. This allows authors to revert a page to an older version to rectify a mistake, or counteract a malicious or inappropriate edit. These stores are typically presented for each page in a list called a "log" or "edit history". A diff feature may be available, which highlights the changes between any two revisions.

The edit history view in many wiki implementations will include edit summaries written by users when submitting changes to a page. Similar to a log message in a revision control system, an edit summary is a short piece of text which summarizes and perhaps explains the change.

Wikis offer free navigation between their pages via hypertext links in page text, rather than requiring users to follow a formal or structured navigation scheme. Users may also create indexes or table of contents pages, hierarchical categorization, or other forms of content organization. Most wikis allow the titles of pages to be searched, and some offer full text search of all stored content.

Beginning with the WikiWikiWeb in 1995, most wikis used camel case to name pages, where words in a phrase are capitalized and the spaces between them removed (e.g. "CamelCase"). Any instance of a camel case phrase would be transformed into a link to another page named with the same phrase.

To avoid the downsides of camel case, the syntax of wiki markup gained free links, wherein a term in natural language could be wrapped in special characters to turn it into a link without modifying it. In UseModWiki, link terms were wrapped in double square brackets, for example [[Kingdom of France]]. This syntax was adopted by many later wiki software implementations.

It is typically possible for users to create links to pages that do not yet exist, as a way to invite the creation of those pages. Such links are usually differentiated visually, such as being colored red instead of the default blue.

WikiWikiWeb was the first wiki. Ward Cunningham started developing it in 1994, and installed it on the internet domain c2.com on March 25, 1995. Cunningham named it after the Wiki Wiki Shuttle bus at Honolulu International Airport, later observing that "I chose wiki-wiki as an alliterative substitute for 'quick' and thereby avoided naming this stuff quick-web."

Cunningham's system was inspired by his having used Apple's hypertext software HyperCard, which allowed users to create interlinked "stacks" of virtual cards. HyperCard was single-user, and Cunningham was inspired to build upon the ideas of Vannevar Bush by allowing users to "comment on and change one another's text".

Wikipedia became the most famous wiki site, launched in January 2001 and entering the top ten most popular websites in 2007. In the early 2000s, wikis were increasingly adopted in enterprise as collaborative software. On March 15, 2007, the word wiki was listed in the online Oxford English Dictionary.

In the late 1990s and early 2000s, the word "wiki" was used to refer to both user-editable websites and the software that powers them.

By 2014, Ward Cunningham's thinking had evolved, leading him to argue that the word "wiki" should not refer to a single website, but rather to a mass of user-editable pages or sites, so that a single website is not "a wiki" but "an instance of wiki". This concept relates to wiki federation, in which the same content can be hosted and edited in more than one location.

The software which powers a wiki may be implemented as scripts which operate an existing web server, a standalone application server, or in the case of personal wikis, a standalone application on a single computer. Some wikis use flat file databases to store page content, while others use a relational database, as indexed database access is faster on large wikis.

Wikis can be created on wiki hosting services (also known as wiki farms), where the server-side software is implemented by the wiki farm owner. Some hosting services offer private, password-protected wikis requiring authentication to access.

The four basic types of users who participate in wikis are readers, authors, wiki administrators, and system administrators. System administrators are responsible for the installation and maintenance of the wiki software. Wiki administrators maintain content and are granted additional functions such as preventing edits to pages, deleting pages, changing users' access rights, or blocking them from editing.

Wikis are generally designed with a soft security philosophy in which it is easy to correct mistakes or harmful changes, rather than attempting to prevent them. Most wikis offer a recent changes page which shows recent edits. The version history feature allows harmful changes to be reverted quickly.

Some wiki software provides additional content control. A watchlist feature alerts a person maintaining pages of modifications to them. Some wikis also implement patrolled revisions or flagged revisions, where edits must be reviewed before going live.

Critics of publicly editable wikis argue that they could be tampered with by malicious individuals or unskilled users who introduce errors. Proponents maintain that these issues will be caught and rectified by a wiki's community of users.

Trolling and cybervandalism on wikis can be a major problem. Larger wikis may employ bots that automatically identify and revert vandalism. For example, on Wikipedia, the bot ClueBot NG uses machine learning to identify and revert likely harmful changes within seconds.

Disagreements between users over content may cause edit wars, where competing users repetitively change a page back to a version that they favor. Some wiki software allows administrators to prevent pages from being editable until a decision has been made.

The English Wikipedia has the largest user base among wikis on the World Wide Web. Other large wikis include the WikiWikiWeb, Memory Alpha, and Wikivoyage. Many wiki communities are private, particularly within enterprises, often used as internal documentation for in-house systems and applications.

Wikis have been used in the academic community for sharing and dissemination of information across institutional and international boundaries. They have also found use within the legal profession and government. Examples include the Central Intelligence Agency's Intellipedia, designed to share and collect intelligence assessments, and the United States Patent and Trademark Office's Peer-to-Patent, a wiki to allow the public to collaborate on finding prior art relevant to pending patent applications.

A city wiki or local wiki is a wiki used as a knowledge base and social network for a specific geographical locale. Such wikis contain information about specific people, places, and establishments in a local area.

Joint authorship of articles can cause editors to become tenants in common of the copyright, making it impossible to republish without permission of all co-owners. Some copyright issues can be alleviated through the use of an open content license, such as the GNU Free Documentation License or Creative Commons licenses.

Wikis and their users can be held liable for certain activities that occur on the wiki. In the United States, wikis may benefit from Section 230 of the Communications Decency Act, which protects sites that engage in "Good Samaritan" policing of harmful material.

• Comparison of wiki software
• Content management system
• List of wikis
• Mass collaboration