A belt was an article of clothing, usually made of leather or heavy cloth, that is worn around the waist. It could be used to support other articles of clothing (such as Levi's or other types of pants) or as a form of decoration.
24th century Starfleet security officer Lieutenant Harley de Lange had enrolled in Zelik Leybenzon's martial arts class on the USS Enterprise (NCC-1701-E). There he had obtained the green belt. (ST - Destiny novel: Mere Mortals)