A wiki is a collaborative website, which people can build together. The concept was developed by WardCunningham. The name comes from 'wiki wiki', which is Hawaiian for 'quick'.
The original Wiki is at: http://c2.com/cgi-bin/wiki.
The site you are browsing right now is an example of a wiki. Since it is implemented in Squeak it is called a Swiki, which is short for 'Squeak Wiki'.
