A species of atoms; all atoms with the same number of protons in the atomic nucleus.
A pure chemical substance composed of atoms with the same number of protons in the atomic nucleus. Sometimes this concept is called the elementary substance as distinct from the chemical element as defined under 1, but mostly the term chemical element is used for both concepts.
Source: Red Book, 3rd ed., p. 35 (https://doi.org/10.1515/pac-2014-0718)