Retaliation
Retaliation refers to the counterattack of a stack that has been the target of a melee attack. All creatures have one retaliation per combat round with the exceptions of several creatures: Griffins, which have two retaliations per combat round; Royal Griffins and Fangarms (available in the Horn of the Abyss expansion), which have an unlimited number of retaliations per combat round. With the Counterstrike spell, it is possible for a hero to magically add one or two extra retaliations per combat round. For Griffins this means three or four in total.
Some creatures have special ability typically phrased as "no enemy retaliation", which does not allow the stack(s) they attack to retaliate against them. This also means, that the attacked stack will not lose one of their retaliations for that combat round if attacked by a creature stack with no enemy retaliation. Thus, while attack with no retaliation is a very strong ability, sometimes it's a double-edged sword as creatures with no enemy retaliation cannot be used to prevent an enemy creature from retaliating that round.
Creatures with no enemy retaliation:
Sprite 
Harpy Hag
Cerberus
Vampire and Vampire Lord
Psychic Elemental and Magic Elemental 
Naga and Naga Queen
Devil and Arch Devil
Hydra and Chaos Hydra
Corsair and Sea Dog 
Creatures with more than one retaliation per combat round:
Griffin has two retaliations
Royal Griffin has unlimited retaliations
Fangarm has unlimited retaliations
Creatures causing modified retaliation:
Scorpicore can paralyze on attack which prevents creatures from acting. The next attack on that creature will only be retaliated against at 25% strength
Basilisk / 
Greater Basilisk can petrify on attack which prevents creatures from acting. The next attack on that creature will do 50% damage
Medusa / 
Medusa Queen can stone gaze on attack which prevents creatures from acting. The next attack on that creature will do 50% damage
Unicorn / 
War Unicorn can 
Blind on attack which prevents creatures from acting.
