Mage Armor

You touch a willing creature who isn't wearing armor. Until the spell ends, the target's base AC becomes 13 plus its Dexterity modifier. The spell ends early if the target dons armor.

You touch a willing creature who isn't wearing armor, and a protective magical force surrounds it until the spell ends. The target's base AC becomes 13 + its Dexterity modifier. The spell ends if the target dons armor or if you dismiss the spell as an action.