A compound present in a standard sample of known concentration and volume which is
analysed separately from the unknown sample under identical conditions. It is used
to facilitate the qualitative indentification and/or quantitative determination of
the sample components. The volume of the external standard (standard sample) need
not to be known if it is identical to that of the unknown sample.
PAC, 1993, 65, 819
(Nomenclature for chromatography (IUPAC Recommendations 1993))
on page 837