heraldic insignia or markings on armor (as in a coat of arms). These arms are hereditary; just because someone with your surname once had a coat of arms, does not mean that you, or everyone with your surname, is entitled to use those arms.
The heraldic device of someone who has an award of arms or better is his or her arms; other people have devices, in the SCA and SGU, whereas in the real Middle Ages they had nothing at all; commoners didn't have heraldic devices of any kind. In the SCA and SGU the lowest level of arms is an award of arms; above that is a grant of arms, commonly conferred on Barons; while the noble orders of the Spur, the Laurel, the Pelican, and the rank of Count/Countess and Duke/Duchess confer a patent of arms.