TexasCHLforum.com

...A criminal adversary must have, or reasonably appear to have:

the ability to inflict serious bodily injury (he is armed or reasonably appears to be armed with a deadly weapon),...

Disparity of force comes into play as well. A woman confronted with a larger man. A victim confronted by a group of attackers. An elderly or disabled person facing a younger/stronger person, etc. The attacker(s) does not have to have a weapon to have the ability to do serious bodily injury. But a jury must believe it was reasonable. Zimmerman is sweating this right now in his trial. Good article.