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Species

Definition:

A group of organisms that differ from all other groups of organisms and that are capable of breeding and producing fertile offspring. This is the smallest unit of classification for plants and animals.

Source: OceanLink Glossary of Common Terms and Definitions in Marine Biology 

More:

An interbreeding group of organisms that is reproductively isolated from all other organisms, although there are many partial exceptions to this rule in particular taxa. Operationally, the term species is a generally agreed fundamental taxonomic unit, based on morphological or genetic similarity, that once described and accepted is associated with a unique scientific name.

Source: Millennium Ecosystem Assessment  Glossary

Related words:

Alien species - Endangered species

To read about this term in context:

GreenFacts Summary on Biodiversity & Human Well-being:

Translation(s):

Deutsch: Art (Biologie)
Español: Especie
Français: Espèce
Nederlands: Soort

Related publications: