My instructor said I can't use Wikipedia. Is this true? Why?
Wikipedia is "the free encyclopedia that anyone can edit."
This is both its strength and its weakness.
There is a lot of great information in Wikipedia, but the problem is that ANYONE can edit its information...for both good and bad.
The reason why that concerns faculty is that one cannot be certain that factual data retrieved there is correct. It is also difficult to assess the authority of the authors (and authority is a big thing in the academic world!).
Instead of using Wikipedia, per se, examine the sources cited in Wikipedia (scroll down to the very bottom of the article). Ask yourself: are any of theses sources authoritative and credible? If so, find them/link to them, read the information there, evaluate it, and if appropriate, use information from that source instead of the Wikipedia article.
You might also (since Wikipedia is a type of encyclopedia) try using the print and electronic encyclopedias and dictionaries available via the library. Try using CREDO eReference Books and GALE eReference Books as starting points.