Credo, Gale, and Oxford offer reference collections that let you cross-search many dictionaries and encyclopedias at once!
Online reference works from a variety of publishers.
Reference books by Oxford University Press, including the Oxford Digital Reference Shelf.
Not sure if it's okay to use Wikipedia or a web search as a starting point for your research? Check with your instructor or a librarian!
For an overview of the issue, check out Wikipedia's Citing Wikipedia article!