In scientific classification used in biology, the order (Latin: ordo, plural ordines) is a rank between class and family (termed a taxon at that rank). The superorder is a rank between class and order: the superorder is above the order. Exact details of formal nomenclature depend on the Nomenclature Code which applies, see scientific classification and:
History of the concept
The order as a distinct rank of biological classification having its own distinctive name (and not just called a higher genus (genus summum)) was first introduced by a German botanist Augustus Quirinus Rivinus in his classification of plants (appeared in a series of treatises in the 1690s). Carolus Linnaeus was the first to apply it consistently to the division of all three kingdoms of Nature (minerals, plants, and animals) in his Systema Naturae (1735, 1st. Ed.).
It should be noted that for plants the Linnaean orders, in the Systema Naturae and the Species Plantarum, were strictly artificial, introduced to subdivide the artificial classes into more comprehensible smaller groups. When the word ordo was first consistently used for natural units of plants, in nineteenth century works such as the Prodromus of de Candolle and the Genera Plantarum of Bentham & Hooker, it indicated taxa that are now given the rank of family (see ordo naturalis).
In French botanical publications, from Michel Adanson's Familles naturelles des plantes (1763) and until the end of the 19th century, the word famille (plural: familles) was used as a French equivalent for this Latin ordo. This equivalence was explicitely stated in the Alphonse De Candolle's Lois de la nomenclature botanique (1868), the precursor of the currently used International Code of Botanical Nomenclature.
In the first international Rules of botanical nomenclature of 1906 the word family (familia) was assigned to the rank indicated by the French "famille", while order (ordo) was reserved for a higher rank, for what in the nineteenth century had often been named a cohors (plural cohortes).
Some of the plant families still retain the names of Linnaean "natural orders" or even the names of pre-Linnaean natural groups recognised by Linnaeus as orders in his natural classification (e.g. Palmae or Labiatae). Such names are known as descriptive family names.
In zoology, the Linnaean orders were used more consistently. That is, the orders in the zoology part of the Systema Naturae refer to natural groups. Some of his ordinal names are still in use (e.g. Lepidoptera for the order of moths and butterflies, or Diptera for the order of flies, mosquitoes, midges, and gnats).