Device

A device is the symbol of a person or group. This is what people paint onto their shields or sew into their clothing (or weave into their Chainmaille, etc). Devices are a big part of Heraldry

The SCA has some standards for devices (if you plan to have it registered) and you'd do well to consult a Herald before you get to bogged down in ideas (although its fine to work on your own if you choose).