Since dictionaries do not list authors, the in-text citation gives the name of the dictionary you used as a reference along with the year of publication with a comma between the two. The title of the online dictionary appears in italics and should be spelled out unless the title is very long. You can shorten it using the first word and a few other words from the title if necessary. For instance, The American Dictionary of Medical and Scientific Terminology in the 21st Century could be shortened to American Dictionary of Med Terms. A typical citation might look like this example: (Merriam-Webster's online dictionary [italicized], 2012).
Unlike other anonymous sources, the References page entry for an online dictionary does not begin with the information you include in the in-text citations. Instead, the entry starts with the word from the dictionary, followed by a period. Put the publication date of the page in parentheses with a period after it, or put "n.d." without the quotation marks in the parentheses if there is no date listed. The name of the online dictionary comes after "In" (without the quotation marks), italicized, ending with a period. Capitalize the first word of the site and any proper nouns. Then type "Retrieved from" (without the quotation marks) and the full URL. An entry would look like this:
Mania. (n.d.). In Merriam-Webster's online dictionary [italicize the title]. Retrieved from http://www.merriam-webster.com/dictionary/mania
If you access an online version of a print dictionary, include the edition in parentheses after the title but before the period:
Mania. (n.d.). In Merriam-Webster's online dictionary [italicize the title] (11th ed.). Retrieved from http://www.merriam-webster.com/dictionary/mania