A shield (as opposed to a Buckler) is a large shield that you can either strap to your arm or hold by a grip.
A Shield must be more than 16 inches at its smallest diameter; at its widest diameter, the Shield must be no more than 36 inches across.
You can only wield a Shield this size if you have one of the Myrmidon or Abjurer Abilities.
Any attack or projectile that hits a Shield while you are actively wielding it deals no damage. Some Called effects (such as Crush and Repel) may still affect you even if they strike your Shield, but even then you take no damage from the hit.
You can wield a Shield in each of your hands, if you wish.
Any arm that you are wielding a shield with cannot be used for anything else, such as wielding a weapon or throwing Evoker packets.
A Shield that is strapped to your back (or any part of you other than your forearm) does not block attacks; any attack that hits it counts as hitting you.