1 An arrangement or sequence of objects or elements; or an established or customary social system or structure, especially a social hierarchy.
2 A command or instruction issued by a person in a position of authority over another.
3 In biology, a taxonomic group into which a class (2) is divided, containing one or more families (2). Thus the class of Mammalia (mammals) includes the orders Carnivora and Primates, each of which is divided into families. [From Latin ordo an order]