It is not unusual for people to want to add wiki functionality to their website. There is no ‘wiki’ module in the Drupal.org repository. Drupal is often described as a content management platform (as opposed to a CMS) because it provides the tools for building a site that the developer then assembles.
This is a video tutorial I created that provides a walk-though for setting up a wiki in Drupal 7. A number of custom modules are used in the tutorial to build a fully functional wiki.