||This article needs attention from an expert on the subject. (July 2010)|
- 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|
|Parvclass||parvus: small, unimportant||Neornithes|
An example from botany 
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History 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 (it appeared in his 1694 Eléments de botanique).
Carolus Linnaeus, in his Systema Naturae (1735, 1st ed.). divided of 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.