Colloquially, a wiki is a term used to describe a website that features content created, edited, and maintained collaboratively. Wikis are accessed and modified through web browsers and so make it really accessible to anybody with an internet connection.
The term "wiki" comes from Ward Cunningham, the creator of the wiki software and the wiki software engine. Cunningham named his software "wiki" after his memories of the Wiki Wiki Shuttle at the Honolulu International Airport . The actual word "wiki" is the Hawaiian word for "quick".
The origin of the term "wiki" is important because the whole mentality of wiki-sites revolves around them being quick to access, modify and update.
Ward Cunningham in his book, "The Wiki Way: Quick Collaboration on the Web", describes the fundamental philosophy behind wikis. Specifically, the essence of the wiki concept is as follows:
- A wiki invites all users to edit any page or to create new pages within the wiki Web site, using only a plain-vanilla Web browser without any extra add-ons.
- Wiki promotes meaningful topic associations between different pages by making page link creation almost intuitively easy and showing whether an intended target page exists or not.
- A wiki is not a carefully crafted site for casual visitors. Instead, it seeks to involve the visitor in an ongoing process of creation and collaboration that constantly changes the Web site landscape. 
Information is the most important part of a wiki and its collaborative nature allows for all users to contribute. This barrier to entry is much lower than getting a book or paper published and allows anybody with relevant information to contribute. This more often a blessing than a curse because wiki software allows for unwanted or irrelevant changes to be easily reverted to the original state of an article.
The first site that could be considered a "wiki" by today's standards was WikiWikiWeb created by Ward Cunningham in 1994/1995 . Over the next few years the size and popularity of WikiWikiWeb approximately doubled with the addition of new features. WikiWikiWeb was focused on programming principles and topics and so its popularity was not felt outside of programming circles.
Wikis did not hit the mainstream until about 2001 when Wikipedia was launched. Content on Wikipedia was not limited to programming topics and thus it attracted much more attention. At the conclusion of its first year, Wikipedia had approximately 20,000 articles written. Over the coming years, the term "Wikipedia" and more generally "wiki" would become household terms.
A big milestone for wikis was in March 2007 when the word "wiki" was entered into the Oxford English Dictionary. In addition to this, Wikipedia was named one of the top 10 popular websites in the world in 2007. Wikipedia had become a household name.