In biology, a  is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity.

A is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction.

Other ways of defining include their karyotype, DNA sequence, morphology, behaviour or ecological niche.

In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined.