A Critical Effect is when a damaging or healing spell, or ranged or melee attack, strikes in an especially effective way. Typically this means twice the normal amount of healing or damage is dealt, though this may be modified by many things. This is described as a Critical Hit when triggered by a physical attack, a Critical Heal when triggered by a healing spell, and just a Critical Effect when triggered by a damaging spell.
In Warhammer Online, there are three statistics which add bonuses to a character's chance to create one of these effects. These are: Weapon Skill for melee attacks, Ballistic Skill for ranged attacks, and Intelligence for magical attacks. It is uncertain whether Intelligence also affects the chance to generate a Critical Heal.
Many careers have abilities which modify their chance to generate a Critical Effect, but the most notable are the Combustion and Dark Magic specialties of the Bright Wizard and Sorceress respectively.