Ferocity: An isolated, static ability that means, "Damage from this unit can't be prevented." Damage that can't prevented includes all damage coming from the unit with Ferocity, including damage from attacks, Ambush, Deflect, Retaliate, and other abilities. Units with Fortitude override this ability, and can prevent damage to themselves from units with Ferocity.
Persuade
Persuade X: An activated ability that means, "Prevent up to X damage to this unit, and the attacking unit, if any, loses power for its next attack equal to the amount of damage prevented this way."