Fortitude defense is based on Stamina and measures health and resistance to threats like poison or disease. It incorporates constitution, ruggedness, metabolism, and immunity.
Defenses are also used to measure the ability to overcome certain effects, involving a resistance check of the defense plus a die roll against a difficulty class determined by the effect or hazard. So you might make a Fortitude resistance check for your hero to overcome a toxin, for example, or a Dodge resistance check to avoid a trap just as it is triggered, and so on. This is referred to as “resisting,” such as “resisted by Fortitude” or “resisted by Dodge”.
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