Chemical species are atoms, molecules, molecular fragments, ions, etc., as entities being subjected to a chemical process or to a measurement. Generally, a chemical species can be defined as an ensemble of chemically identical molecular entities that can explore the same set of molecular energy levels on a characteristic or delineated time scale.
The original article was at Chemical species. The list of authors can be seen in the page history. As with Chemistry, the text of Wikipedia is available under the GNU Free Documentation License.