Passive Spells are spells, usually found in a Class' Support Spells, that are simply "always on" during combat. They require no casting cost, or targeting, etc. They usually provide no bonus if left at level 0, and the bonuses usually provide benefits that specifically match the class. Most Passive Spells available to the classes have no effect if at spell level 0, so must be leveled at least once to provide any benefit.