If your definition was retrieved online, you will cite it like any web site:
Entry Name. (Year, Month Date). In Website title (edition). Retrieved from (URL)
Heuristic. (n.d.). In Merriam-Webster’s online dictionary (11th ed.). Retrieved from http://www.m-w.com/dictionary/heuristic
A definition retrieved from a print source would be cited like a standard book chapter citation:
Term. (Date). In Title of book (edition). Place of publication: Publisher.
Jejunum. (2003) In Merriam-Webster's dictionary (11th ed.). Springfield, MA: Merriam-Webster, Inc.
To cite a definition within the text, you would place the defined word and the date of publication in parentheses after the relevant phrase and before the punctuation mark. If the definition is quoted, you must also add the page number.
According to Merriam-Webster's dictionary, andragogy is "the art or science of teaching adults"... ("Andragogy", 1993, p. 85).
For additional help with using APA style, see the OWL at Purdue web site linked below.