Dodge defense is based on Agility rank. It includes reaction time, quickness, nimbleness, and overall coordination, used to avoid ranged attacks or other hazards where reflexes and speed are important.
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