According to this video Path of Exile: Critical Strike Guide by subtractem, a critical strike is a skill or default attack that deals increased damage and can potentially inflict elemental ailments.
Here’s a summary of critical strikes in Path of Exile 2:
Critical strikes are rolled on a per-action basis, not per target. This means that if you shoot a fireball with greater multiple projectiles, all of the fireballs will either crit or not crit at the same time.
The chance to deal a critical strike is taken from the weapon used to perform the attack or the spell itself. For spells, the chance is listed in the skill gem description.
Weapons that have local critical strike chance, such as a dagger that may have 6% critical strike chance, do not affect the chance to score a critical strike chance with a spell.