A heraldic badge, emblem, impresa, device, or personal device worn as a badge indicates allegiance to, or the property of, an individual, family or corporate body. Medieval forms are usually called a livery badge, and also a cognizance. They are para-heraldic, not necessarily using elements from the coat of arms of the person or family they represent, though many do, often taking the crest or supporters. Their use is more flexible than that of arms proper.
The Dunstable Swan Jewel, a livery badge from about 1400 AD, perhaps of Henry V as Prince of Wales. British Museum