Question

What is the best biological definition of species

Answer 1

One of the basic units of biological classification and a taxonomic rank. 

Answer 2

A group of closely related organisms that are very similar to each other and are usually capable of interbreeding and producing fertile offspring. The species is the fundamental category of taxonomic classification, ranking below a genus or subgenus. Species names are represented in binomial nomenclature by an uncapitalized Latin adjective or noun following a capitalized genus name, as in Ananas comosus, the pineapple, and Equus caballus, the horse.
2. Logic A class of individuals or objects grouped by virtue of their common attributes and assigned a common name; a division subordinate to a genus.
3. Chemistry A set of atoms, molecules, ions, or other chemical entities that possess the same distinct characteristics with respect to a chemical process or measurement.