Just what is a wiki, anyway? A few years ago, I could have told you. Today, I'm not so sure.
Part of the problem is that wiki authors have made innovations, broadening the appeal and scope of wikis. Witness MindTouch's recent upgrade of its DekiWiki platform to 1.8.3.
DekiWiki, the leading enterprise wiki (measured in terms of downloads and, in my opinion, by raw innovation), still allows easy creation of Web pages by nontechnical users.