Why do we need to know that?
NO self-respecting educator would allow students to cite Wikipedia in a research paper!
NO professional would try to cite Wikipedia in a refereed journal!
Why waste the time and effort to even consider how to cite it?
It's funny how the title of this posting will raise the dander of many an academic. It will cause them to respond exactly as the quotes above indicate. Many an academic will complain that Wikipedia is an indicator of how far our world has plummeted when they consider that a resource created "by the masses." It is "the free encyclopedia that anyone can edit."
This reminds me of a little known fact that Dr. David Thornburg mentioned once in a presentation I had the good fortune to enjoy. He was addressing this very issue of Wikipedia being considered unreliable because it was created and maintained by people who were not christened as "experts" in the field. He pointed out that the first Oxford Dictionary had been compiled by 800 volunteer readers back in the mid-1800s.
Many question Wikipedia's reliability. It can be edited by anyone with an account. Most teachers and publications don't accept it as a valid reference. I don't accept Wikipedia as a valid reference from my students.
But ask a room full of academics and educators about how many of them use Wikipedia and the only liars in the room will be the ones who don't have their hands raised. Wikipedia is a WONDERFUL starting place for beginning research or getting familiar with a topic or even finding relevant references in the References and External Links at the end of each article.
Run a Google search on most any topic and there will be at least one Wikipedia reference in the first 10 links. Wikipedia is ubiquitous. It is everywhere.
So what do you do if you want to refer to a definition in Wikipedia (not necessarily as an expert resource but as an example in a discussion) and you want to make an APA-appropriate citation? Listen to Timothy McAdoo in the APA Style Blog. His posting, How to Cite Wikipedia in APA Style, explains that it is quite similar to any other electronic citation with a couple of modifications to make it Wikipedia-specific:
Infinite Monkey Theorem. (n.d.). In Wikipedia. Retrieved December 9, 2009, from http://en.wikipedia.org/wiki/Infinite_Monkey_Theorem
Give it a try. Do you use Wikipedia? What is your call on using it as a reference tool?