(singular and plural))biologically, a group of populations of actually or potentially interbreeding organisms which are reproductively isolated (by behavior, ecology, morphology or physiology) from other such groups. Historically, a cohesive lineage of ancestral-descendent populations of organisms that maintains its identity from other such lineages. Taxonomically, the name of a category of organisms below the genus-group; an individual taxon of the category "species", e.g. Perca fluviatilis. Morphologically, the smallest group of animals that can always be separated (+95% of the specimens or all of the specimens) from other such groups by morphological characters (other than sex or individual genes) – does not include sibling species. A species is a subjective unit insofar as it is based on only a sample of the population and insofar as the point of separation where there is some overlap must be arbitrary. Abbreviated as sp. (singular) spp. (plural). A nominal species is a named species, objectively defined by its type-specimen
Dictionary of ichthyology. 2009.