A wiki (the Hawaiian word for 'quick'), is a collaborative web site that allows anyone to edit its pages. This seems like a bizarre concept to most people who aren't used to editing the web sites they visit.
In fact, the web's creators first imagined it as a collaborative space where web site users could literally edit page content as they were browsing it. Somewhere along the way this vision was lost.
In 1995, a programmer named Ward Cunningham wrote a script he called a wiki that allowed anyone to edit his web site (which, if you're interested, is all about programming patterns). This idea flourished, and before long, he had an enormous web site about programming patterns... easily the biggest and most comprehensive of its kind on the Internet.
Since then, many have copied the wiki concept and taken it further. Wikipedia, the most widely known wiki, is a vast online encyclopedia written end meticulously edited by anonymous visitors to the site.