An entry in a reference book such as an encyclopedia is cited as a chapter in an edited book. If the reference work has more than one volume, then the volume number goes immediately before the page numbers.
Template:
Author, A. A. (Year). Title of article: Subtitle of article. In E. E. Editor (Ed.), Title of reference book: Subtitle of reference book (Vol. X, pp. xx-xx). ...
Example:
Bukowski, P. (2012). Basics of quantitative equity investing. In P. P. Drake & J. Fabozzi, J.(Eds.) Encyclopedia offinancial models (Vol. II, pp. 89-106). Retrieved from http://www.ebrary.com