According to Wikipedia, the world largest wiki site:
A Wiki ([ˈwiː.kiː] <wee-kee> or [ˈwɪ.kiː] <wick-ey>) is a type of website that allows users to add, remove, or otherwise edit and change most content very quickly and easily.
And that is it! As a part of Wikidot.com network this Site is a customizable piece of the Internet where Users can edit content, upload files, communicate and collaborate thus it wont be wrong to state:
"A wiki is a single web page or a collection of web pages that are specifically designed in such a way that anyone who accesses them can contribute new content or modify the existing content by using a simplified markup language."
In other words we can say that Wiki site is basically a type of website which can be created and edited by the common web users.