Q. Do I need to cite for definitions in APA format?

Yes. Anything you did not already know for which you consulted a source should be cited. Here is an example from OWL:

Merriam-Webster’s collegiate dictionary (10^th ed.).(1993).

Springfield, MA: Merriam-Webster.

The Merriam Webster site provides an online example:

hacker. 2011. In Merriam-Webster.com.Retrieved May 8, 2011,

from http://www.merriam-webster.com/dictionary/hacker