Barkskin

Barkskin is a transmutation spell that toughens the skin to a bark-like consistency.

Effects
Barkskin makes a creature's skin as tough as bark. The effect grants a +3 natural armor bonus to AC. This bonus increases to +4 at 6th level and to +5 at 12th level and up.

Since the AC bonus is a natural armor bonus, it does not stack with any natural armor the subject may already have.

Source
3rd edition 4th edition