Patronus

A Patronus is the guardian produced by Expecto Patronum. A non-corporeal Patronus is a silver shield. A corporeal one takes an animal shape tied to the caster: a stag for Harry, an otter for Hermione, a terrier for Ron, and a doe for Snape. Dementors recoil from it. It can also be sent as a message, which the Order uses.