From WhatIsaWiki (edited):

A Wiki is a collaboratively-edited website. The distinguishing feature of wikis is typically allowing all users to edit any page, with full freedom to edit, change and delete the work of previous authors. Other typical wiki features include:

Wikis use revision control to prevent abuse and track changes, so if someone decides to erase one or more pages it is easy to "revert" back to previous good versions of the "erased page".