"Critical damage" is one of the outcomes of the Attack Dice. It counts as rolling regular Damage on two dice.
Damage
Damage is the general term for effects that reduce the health points of a target, and it is also the name of one of the sides that can come up when rolling Attack Dice used when a player attacks an enemy, and when rolling Enemy Dice when an enemy is attacking a Crew member or a ship. The "Damage" dieroll outcome results in dealing 1 damage to the target, unless the damage is cancelled by some effect (for example via Evasion).
Damage caused to an enemy is tracked by placing Trauma markers (the red acrylic gems) on the corresponding enemy card. If the number of Trauma markers equals or exceeds the enemy's health, the enemy is destroyed.
Damage caused to a Crew member is tracked by placing Trauma markers on the corresponding Crew Board. If the Trauma/Strain slots on the Crew Board become full, the Crew member dies, unless some Trait or other effect prevents it.
Damage caused to a player's ship is tracked by rolling the Fate Die once for each damage, and placing a Destroyed token on the Node in that Zone on the Ship Board that matches the number rolled on the Fate Die. If the Nodes in that Zone are already Destroyed, instead place the marker on the Node in the nearest lower numbered Zone, or if there is none, on the nearest higher numbered Zone, where there is a non-Destroyed Node. Destroyed Nodes can be repaired using the Repair Service when you Visit a Port.
If all Nodes of a Ship Board are damaged, the ship is sunk.