You can cite Wikipedia in APA style just like any other web page for which there is no author. For an article with no author, use the title of the article as the reference. In your reference section, give the full details of the citation beginning with the title. (The word used in the in-text reference must match the first word of the reference, which is in alphabetical order, so the reader can easily find the reference if desired).
Often teachers don't want you to use Wikipedia because it is publicly edited. That means right now I could go on it and add Bozo the Clown to the list of Presidents of the United States. But, Wikipedia has tools to deal with such malicious changes. First, authorized editors are notified automatically about changes, so they will quickly remove any deliberate damage. Second, they will ban me from further editing. So things are quickly returned to normal. Chances are that any citation from Wikipedia is correct.
However, for a research essay, you should be using peer-reviewed journal articles. This means that the information has been written and reviewed by experts in the field. Wikipedia is not a peer-reviewed source. So you probably shouldn't use it for a university-level paper. The point of a research paper is to do research. That means trying to find the original source of the information. Wikipedia is, at best, a secondary source. But it has its uses.
I often use Wikipedia to review the basic facts for some essay I am editing. When someone writes about the politics of Argentina and their grammar is so full of errors that I have difficulty understanding what they are saying, or even that there are two possible meanings, I might consult Wikipedia to get a basic understanding of the situation. In addition, Wikipedia can be useful for general definitions, just like you would use an encyclopedia or dictionary.
The APA style blog also warns against using Wikipedia. However, if you do use Wikipedia, they have a format. A recent client was writing about disaster planning and needed a basic definition of "social issues" to help her discuss the definition of "disaster." For the in-text citation, she put the title of the article and the date ("Social issue," n.d.). The title of the article is in quotation marks, and there is no specific date cited for the article. Notice that she uses the title of the article for both the in-text citation and for the reference list. This is because these two must be identical in all citations so the reader can easily find the reference in the reference list if they want to follow up on the citation.
Finally, here is her reference list citation:
Social issue. (n.d.) In Wikipedia. Retrieved (insert date here), from http://Wikipedia.com/wiki/Social_issue
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