Spell failure is the chance that a spell fails and is ruined when cast under less than ideal conditions; when a spell is cast to no effect. It occurs when spellcasting is attempted in conditions where the characteristics of the spell cannot be made to conform, the casting fails and the spell is wasted. The failed spell's associated spell slot or use per day is considered to be used.
Spell failure occurs when the caster fails his or her concentration check that is required when the caster is distracted in some manner during casting (e.g., incurring damage) or when casting defensively. Spell failure in this manner is guaranteed and can affect both divine and arcane spellcasters.
Arcane spellcasters receive a chance of spell failure when wearing armor or wielding a shield that has a percent value for arcane spell failure. The overall chance of failure incurred by wearing armor and/or using a shield is the cumulative total of arcane spell failure values of the worn armor and/or wielded shield. The chance of spell failure is cumulative with that of being taunted. Some armors and shields possess a property that reduces the chance of arcane spell failure. This method of failure only applies to arcane spells with somatic components. Divine spells cannot fail in this manner.