Defense is a skill tree in Ars Magica 2, found in the Occulus, along with Utility and Offense, the other active skill trees, and Talents, which is a passive skill tree. It is used for, as the name implies, more defensive spells, including things like regeneration, healing, or gravity, slow fall, even shield, if you go far enough.
The tree includes the following, from top (easier to get) to bottom (harder to get).
Shapes: Self, Contingency: Fall, Contingency: Damage, Wall, Zone, Contingency: Health, and Rune.
Components: Leap, Regeneration, Shrink, Slowfall, Summon, Life Tap, Heal, Slow, Haste, Gravity well, Life Drain, Dispel, Repel, Swift swim, Levitate, Mana drain, Appropriation, Entangle, Accelerate, Flight, Shield, Chrono anchor, and Reflect.
Modifiers: Healing, Speed, Rune Procs, and Duration.
Silver Spells: Mana Link, Mana Shield, and Buff Power.
Some of the more useful items in here are the Shape of Self and Contingency: Damage, the components of Leap, Regeneration, Heal and Shield, and the modifiers of Healing, Speed and Duration, although all of them can be used to great effect if used properly.