The arrangement or classification of objects according to certain criteria. Systematics is a broader term applied to all comparative biology, including taxonomy. For classifying plants and animals, where the term taxonomy is most often applied, the criteria are characters of structure and function.

A given character usually has two or more states. These variations are used as the basis of biological classification, grouping together like species (in which the majority of the character states are alike) and separating unlike species (in which many of the character states are different). Since the acceptance by biologists of the concept of organic evolution, more and more effort has been made to produce systems of classification that conform to phylogenetic (that is, evolutionary) relationships. Taxonomy is thus concerned with classification, but ultimately classification itself depends upon phylogeny—the amount, direction, and sequence of genetic changes. Scientists try to classify lines, or clusters of lines, of descent. This has not always been the case, and in the past various other criteria have been used, such as whether organisms were edible (ancient times) and whether flowers had five stamens or four or some other number (Linnaean times). Modern taxonomists generally agree that the patterns or clusters of diversity they observe in nature, such as the groups of primates, the rodents, and the bats, are the objective results of purely biological processes acting at different times and places in the past. At the least, animal and plant taxonomy provides a method of communication, a system of naming; at the most, taxonomy provides a framework for the embodiment of all comparative biological knowledge.Classification, biological Numerical taxonomy Organic evolution Phylogeny Plant taxonomy