I guess I would still define it by the purpose of the activity. Tetherball isn’t designed for combat, though if you play enough and become physically fit that can help you survive or escape from an assault. Martial practice is designed to develop martial skills. And it ought to help with physical fitness. By the way, I include combat sports as martial arts, such as boxing and wrestling. Even tho they are mostly designed for competition now, it is a competition of combat and their skills are directly transferable to surviving an attack.