The term species has its origins in the ancient Latin language. In this language, the word species means kind. A more technical definition of species is a group of interbreeding organisms that do not ordinarily breed with members of other groups. Biologists estimate that about 10 to 40 million different species inhabit the Earth. Of these species, approximately 1.5 million have been classified.
The first individual to propose an orderly system for classifying the variety of organisms found on our planet was Linnaeus (1753). In his system of classification, the finest unit in the organization of life is the species. Linnaeus suggested that every organism should be classified with a unique binomial name. The first term in this classification system is the organism's generic name or Genus. The second term is the organism's specific name or species designation.
Current classification systems have developed from Linnaeus' original work. However, modern classification systems are much more complicated having many levels of hierarchical organization. These systems are also taxonomic (structural and physiological connections between organisms), phylogenic (classification based on genetic connections between organisms), and are structurally based on Darwin's theory of evolution.
Modern classifications of organisms are standardized in a hierarchical system that go from general to specific. Table 9b-1 below describes the detailed classification of the tree red maple. Note that each level of organization is based on some biological characteristic that the organism possesses.
Table 9b-1: Hierarchical system of the biological classification of an organism.
| Category | Name | Characteristics |
| Kingdom | Plantae | Organisms that usually have rigid cell walls and usually possess chlorophyll. |
| Subkingdom | Embryophyta | Plants forming embryos. |
| Phylum | Tracheophyta | Vascular plants. |
| Subphylum | Pterophytina | Generally large, conspicuous leaves, complex vascular system. |
| Class | Angiospermae | Flowering plants, seed enclosed in ovary. |
| Subclass | Dicotyledoneae | Embryo with two seed leaves. |
| Order | Sapindales | Soapberry order consisting of a number of trees and shrubs. |
| Family | Aceraceae | Maple family. |
| Genus | Acer | Maples and box elder. |
| Species | Acer rubrum | Red maple. |