Encyclopedias, dictionaries and other reference type materials are cited in a similar way to the sections of a book. These reference page entries will usually include:
Reference page entry (unknown author):
Therapy. (2010). In Collins Canadian Dictionary (1st ed., p. 621). Toronto, ON: HarperCollins Publisher.
Note: Use pp. if the entry spans multiple pages; use p. if the entry is only on a single page.
Definitions can be obtained from Google by adding "define:" to the start of your search terms.
However, if you require a definition to use in your assignment, it's better to use a standard web-based or printed dictionary for the purposes of citing. Google draws from a number of sources to create this definition, and it's not always easily to identify their source.
Choose an electronic or printed dictionary from the library, or use a free dictionary online for your definition.
Reference page entry:
Catherine, Duchess of Cambridge. (2014, April 30). In Wikipedia, The Free Encyclopedia. Retrieved, May 2, 2014, from http://en.wikipedia.org/w/index.php?title=Catherine,_Duchess_of_Cambridge&oldid=606514600
Note: Webpages such as Wikipedia are frequently edited. You need to include the date you retrieved the information, just in case the information on the page changes at a later date!