In biology, a species is one of the basic units of biological classification and a taxonomic rank. 

A species is often defined as the largest group of organisms capable of interbreeding and producing fertile offspring. 

While in many cases this definition is adequate, the difficulty of defining species is known as the species problem. 

Differing measures are often used, such as similarity of DNA, morphology, or ecological niche. 

Presence of specific locally adapted traits may further subdivide species into "infraspecific taxa" such as subspecies (and in botany other taxa are used, such as varieties, subvarieties, and formae).

Species hypothesized to have the same ancestors are placed in one genus, based on similarities. The similarity of species is judged based on comparison of physical attributes, and where available, their DNA sequences. 

Binomial naming

All species are given a two-part name, a "binomial name", or just "binomial". The first part of a binomial is the generic name, the genus to which the species belongs. The second part is either called the specific name (a term used only in zoology) or the specific epithet (the term used in botany, which can also be used in zoology). 

For example, Boa constrictor is one of four species of the Boa genus. The first part of the binomial is capitalized, and the second part has a lower case. The binomial is written in italics when printed and underlined when handwritten.

Defining species is important

A usable definition of the word "species" and reliable methods of identifying particular species are essential for stating and testing biological theories and for measuring biodiversity, though other taxonomic levels such as families may be considered in broad-scale studies. 

 Extinct species known only from fossils are generally difficult to assign precise taxonomic rankings, which is why higher taxonomic levels such as families are often used for fossil-based studies.

The total number of non-bacterial and non-archaeal species in the world has been estimated at 8.7 million, with previous estimates ranging from two million to 100 million.