Avenge X: A cumulative, triggered ability that means, "Whenever damage from an attacking unit causes one of your other units in the same arena as this unit to be discarded, this unit does X dice of damage to the attacking unit when the attack ends."
Evade
Evade X: A layered ability activated during the damage prevention POP that means, "Prevent up to X damage to this unit." You can play Evade only once for each instance of damage. You don't have to use all of the Evade. You choose how much damage to Evade up to X, even if the full amount of damage or more is pending.
Ferocity
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.
Fortitude
Fortitude: An isolated, static ability that means, "This unit can prevent unpreventable damage to itself." Units with Fortitude can prevent damage from units with Ferocity and "damage... that can't be prevented" by using Deflect, Evade, Parry, and other damage prevention abilities. Fortitude only allows a unit to prevent unpreventable damage to itself, not to other units. Putting damage counters directly on a unit is not the same thing as doing damage, and also can't be stopped by Fortitude.
Retaliate
Retaliate X: A layered ability activated during the attack POP that means, "If a unit in the same arena as this unit is attacking this unit, this unit does X dice of damage to that unit when the attack ends." A Retaliate ability without an activation cost has a printed cost of "Pay 0 Force -->". Retaliate resolves when the attack ends, even if your unit gets discarded. Retaliate dice are just dice of damage, not attack dice, and can trigger Stun. Retaliate based on a unit’s power is based on its total power at the time the Retaliate is activated. Retaliate doesn't work with attacks from another arena (such as Bombard and Ion Cannon) or the build zone. A unit may retaliate against itself if it attacks itself. Each Retaliate may be used once per attack. Each defending unit can activate Retaliate during the attack POP, which resolve in their order when the attack ends, even if the retaliating unit is discarded.