Online:
Author (surname first). Title of the article. (Date of publication). In Title of the Encyclopedia. Retrieved from [URL]
Example:
Biogeography. (2009). In Britannica Student Encyclopedia. Retrieved from Encyclopædia Britannica Online School Edition: school.eb.com.au/comptons/article-197492
*note: in many cases such as the above example no author name is given.
Print:
Author (surname first). (Date of publication). Title of the article. In Title of the Encyclopedia ([Year or edition] ed., Vol. [volume number], pp [first page of article-last page of article]). Place of publication: publisher.
Example:
Derman, G. H. (2005). Carpal tunnel syndrome. In The World Book Encyclopedia (2005 ed., Vol. 3, pp. 245-246). Chicago: World Book Inc.