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- a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family. An immediately higher rank, superorder, may be added directly above order, while suborder would be a lower rank.
- a taxonomic unit, a taxon, in that rank. In that case the plural is orders (Latin ordines).
What does and does not belong to each order is determined by a taxonomist. Similarly for the question if a particular order should be recognized at all. Often there is no exact agreement, with different taxonomists each taking a different position. There are no hard rules that a taxonomist needs to follow in describing or recognizing an order. Some taxa are accepted almost universally, while others are recognised only rarely.
For some groups of organisms, consistent suffixes are used to denote that the rank is an order. The Latin suffix -(i)formes meaning "having the form of" is used for the scientific name of orders of birds and fishes, but not for those of mammals and invertebrates. The suffix -ales is for the name of orders of vascular plants.
Hierarchy of ranks 
For some clades, a number of additional classifications are sometimes used, although not all of these are officially recognised.
|Name||Meaning of prefix||Example|
|Magnorder||magnus: large, great, important||Epitheria|
|Mirorder||mirus: wonderful, strange||Primatomorpha|
|Parvorder||parvus: small, unimportant||Catarrhini|
In their 1998 classification of mammals, McKena and Beel used two extra levels between Superorders and Order: "Grandorder" and "Mirorder".
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 the German botanist Augustus Quirinus Rivinus in his classification of plants that 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.).
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 19th 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 explicitly 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 19th 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).
See also 
- Biological classification
- Rank (botany)
- Rank (zoology)
- Virus classification