(biology) A species that normally lives and thrives in a particular ecosystem. This can include any species that developed with the surrounding habitat, and can be assisted by or affected by a new species.

(biology) Any species that has been introduced to an environment where it is not native, and that has since become a nuisance through rapid spread and increase in numbers, often to the detriment of native species.

(taxonomy) A grouping of closely-related species that are treated like a single species for practical purposes.

(ecology) Any species of organism that is vulnerable to extinction in the near future

(ecology) A species that exerts a large, stabilizing influence throughout an ecological community, despite its relatively small numerical abundance.

(neologism) The experience of dysphoria associated with the feeling that one's body is of the wrong species.

(taxonomy) A species which has been designated as typical of a genus, and is chosen as the species according to which the genus is named.

(chemistry) An ensemble of chemically identical molecular entities that can explore the same set of molecular energy levels on the time scale of the experiment.

plural of infima species

(logic, philosophy, semantics) The narrowest of species; one that that is not a genus to anything else.