An entry in an on-line reference work is cited in the same way as an entry in any other reference book.
Template:
Author, A. A. (Year). Article title. In E. E. Editor (Ed.), Title of reference wook (Edition information if available). Retreived from http://...
Example:
Driver, J. (2009). The history of utilitarianism. In E. N. Zalta (Ed.), The Stanford Encyclopedia of Philosophy (Summer 2009 Edition). Retrieved from http://plato.stanford.edu/archives/sum2009/entries/utilitarianism-history
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