Order (Latin: ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and recognized by the nomenclature codes. An immediately higher rank, superorder, is sometimes added directly above order, with suborder directly beneath order. An order can also be defined as a group of related families.

What does and does not belong to each order is determined by a taxonomist, as is whether 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 recognized only rarely.

The name of an order is usually written with a capital letter. For some groups of organisms, their orders may follow consistent naming schemes. Orders of plants, fungi, and algae use the suffix -ales (e.g. Dictyotales). Orders of birds and fishes use the Latin suffix -iformes meaning 'having the form of' (e.g. Passeriformes), most orders of insects use the suffix -ptera meaning 'wing', but orders of mammals, reptiles, amphibians and invertebrates are not so consistent (e.g. Artiodactyla, Anura, Crocodylia, Actiniaria, Primates).

For some clades covered by the International Code of Zoological Nomenclature, several additional classifications are sometimes used, although not all of these are officially recognized.

In their 1997 classification of mammals, McKenna and Bell used two extra levels between superorder and order: grandorder and mirorder. Michael Novacek (1986) inserted them at the same position. Michael Benton (2005) inserted them between superorder and magnorder instead. This position was adopted by Systema Naturae 2000 and others.

In botany, the ranks of subclass and suborder are secondary ranks pre-defined as respectively above and below the rank of order. Any number of further ranks can be used as long as they are clearly defined.

The superorder rank is commonly used, with the ending -anae that was initiated by Armen Takhtajan's publications from 1966 onwards.

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. Carl Linnaeus was the first to apply it consistently to the division of all three kingdoms of nature (then minerals, plants, and animals) in his Systema Naturae (1735, 1st. Ed.).