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Life Domain Kingdom Phylum Class Order Family Genus Species
The hierarchy of biological classification's eight major taxonomic ranks. Intermediate minor rankings are not shown.

In biological classification, class (Latin: classis) is:
  • a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the prefix sub-: subclass (Latin: subclassis).
  • a taxonomic unit, a taxon, in that rank. In that case the plural is classes (Latin classes)
The composition of each class is determined by a taxonomist. Often there is no exact agreement, with different taxonomists taking different positions. There are no hard rules that a taxonomist needs to follow in describing a class, but for well-known animals there is likely to be consensus. For example, dogs are usually assigned to the phylum Chordata (animals with notochords); in the class Mammalia; in the order Carnivora.

Hierarchy of ranks

For some clades, a number of alternative classifications are used.

An example from zoology

Name Meaning of prefix Example 1 Example 2 Example 3[1]
Superclass super: above Tetrapoda
Class Mammalia Maxillopoda Sauropsida
Subclass sub: under Thecostraca Avialae
Infraclass infra: below Cirripedia Aves
Parvclass parvus: small, unimportant Neornithes

An example from botanyHistory of the concept

The class as a distinct rank of biological classification having its own distinctive name (and not just called a top-level genus (genus summum) was first introduced by the French botanist Joseph Pitton de Tournefort in his classification of plants that appeared in his Eléments de botanique, 1694.
In the first edition of his Systema Naturae (1735).[2] Carolus Linnaeus divided all three of his kingdoms of Nature (minerals, plants, and animals) into classes. Only in the animal kingdom are Linnaeus's classes similar to the classes used today; his classes and orders of plants were never intended to represent natural groups, but rather to provide a convenient "artificial key" according to his Systema Sexuale, largely based on the arrangement of flowers.

Since then the class was considered the highest level of the taxonomic hierarchy until George Cuvier's embranchements, first called Phyla by Ernst Haeckel,[3] were introduced in the early nineteenth century.