This is the cast type skill for protection spells.
The in game help reads:
Skill duration: random. Type of skill : spell casting skill (arcane skill). Affecting stats: None. It uses no endurance points. Many users of magic who had studied the offensive natures of magic, found that magic of a defensive nature was just as useful. There isn't much that can stand up to sheer power of offensive magic, except perhaps magic itself. There are many spells which deflect, or help one avoid the affects of spell cast upon them. This type of spell requires a certain knowledge of magic and its protective nature. Being skilled in this knowledge can make a caster of protective magic much more capable.