Noun

Singular species

Plural species

species (plural species)

1. A group of plants or animals having similar appearance.

   This species of animal is unique to the area.

2. (biology, taxonomy) A rank in the classification of organisms, below genus and above subspecies; a taxon at that rank
   • Darwin, On the Origin of Species:

     Hence, in determining whether a form should be ranked as a species or a variety, the opinion of naturalists having sound judgment and wide experience seems the only guide to follow.

   I cast the species of the Sun onto a sheet of paper through a telescope.

3. (mineralogy) A mineral with a unique chemical formula whose crystals belong to a unique crystallographic system.
4. (obsolete) The image of something cast on a surface, or reflected from a surface, or refracted through a lens or telescope; a reflection.
5. (Roman Catholicism) Either of the two elements of the Eucharist after they have been consecrated, so named because they retain the image of the bread and wine before their transubstantiation into the body and blood of Christ.

There are quite a lot of definitions of what kind of unit a species is (or should be). A common definition is that of a group of organisms capable of interbreeding and producing fertile offspring of both genders, and separated from other such groups with which interbreeding does not (normally) happen. Other definitions may focus on similarity of DNA or morphology. Some species are further subdivided into subspecies, and here also there is no close agreement on the criteria to be used.

Biologists' working definition

A usable definition of the word "species" and reliable methods of identifying particular species is essential for stating and testing biological theories and for measuring biodiversity. Traditionally, multiple examples of a proposed species must be studied for unifying characters before it can be regarded as a species. It is generally difficult to give precise taxonomic rankings to extinct species known only from fossils.

Some biologists may view species as statistical phenomena, as opposed to the traditional idea, with a species seen as a class of organisms. In that case, a species is defined as a separately evolving lineage that forms a single gene pool. Although properties such as DNA-sequences and morphology are used to help separate closely-related lineages, this definition has fuzzy boundaries. However, the exact definition of the term "species" is still controversial, particularly in prokaryotes, and this is called the species problem. Biologists have proposed a range of more precise definitions, but the definition used is a pragmatic choice that depends on the particularities of the species concerned.