A Critical Hit is an attack which does 50% bonus damage. In addition, any status effects on the hit have a 100% proc rate (unless the target is immune to that effect).
All characters have a base of 1% 
Crit.
- Some classes have higher base crit rates. See Stats comparison for details.
- Many classes offer a 2% bonus to crit rate when they are mastered.
- Various Weapons, Armor, and Accessories will increase your crit rate.
- The Passives Know Weakness and Know Weakness II each increase critical hit rate by 25% and stack with each other.
By default, a crit does +50% damage.
- Wielding a Scythe increases critical damage by 25%. This bonus stacks if you are able to dual wield Scythes (requires Dual Wield and Sturdy Grip).
- The Scoundrel passive Exploit Weakness increases critical damage by 50%.
- If a healing ability can crit and does, its healing will be increased by your critical damage amount. Source
By default, only Regular Attacks can crit.
- If an ability says it "counts as a regular attack" then it can crit.
- The Fellblade's Versatile passive allows Skills, Spells, and Special abilities to crit, both for damage and healing. Source
Boon will allow a skill or spell to crit, even if the character does not have Versatile.- Consumables can never crit, even with the above two abilities.