Parry defense is based on Fighting. It is the ability to counter, duck, or otherwise evade a foe’s attempts to strike you in close combat through superior fighting ability.
Dodge and Parry defenses require a measure of action to be fully effective. Limits on your mobility, focus, and reaction time adversely affect them. If you are vulnerable, your Dodge and Parry defense ranks are halved (divide their normal values by 2 and round up), and if you are defenseless, they are both reduced to 0!
Mutants & Masterminds Hero’s Handbook, Copyright 2011, Green Ronin Publishing; Author Steve Kenson. Chapter-3 Abilities, Pg 54-59
D20Hero SRD Character Creation Article-http://www.d20herosrd.com/character-creation