The AMA Style Insider blog cites a ditionary like this:
Merriam-Webster’s Collegiate Dictionary. 10th ed. Springfield, MA: Merriam-Webster Inc; 1997. (http://blog.amamanualofstyle.com/)

This reference is based on the format for a book found in section 3.12.1 of the manual.

You should include an edition number if one is included as part of a title or is on the copyright page found on the back of the title page of the book.