Is the fundamental unit of taxonic clasifiation and ranking. Although a simple definition, historically it has been the only conceptual consensus among fields given that, in pratice, the are multiple characteristics that set the threshold of how to classify an organism within a species. The set of questions that make "species" difficult to conceptualize is called the Species Problem.
The most conventional defition is this: A species is a group of organisms capable of interbreeding and producing fertile offspring. Although particuarlygood to classify a big set of organisms, this defition has repeatetly arisen the most fundamental of all species questions: How to define the process of interbreeding capability? Many measurements have been implemented to try to answer this question including DNA similarity, morphology, interaction and isolation by niche (allopatric isolation), and behavioral (sympatric) isolation.
To date there are more than 20 defitions of species based in evolution, morphology, phylogenetics, cohesion and genealogy among others. Three of the most important proponents of the optimization of the concept of species were Charles Darwin, Theodosius Dobzhansky and Ernst Mayr; the last two being prominent founders of the modern evolutionary synthesis and strong proponents of defining a species based on reproductive isolation.