CAS Registry Number

A unique number assigned by the Chemical Abstracts Service, a division of the American Chemical Society. Other than being guaranteed unique to a given compound, this number has no particular meaning. CAS Registry Numbers are assigned to every uniquely identifiable substance, so ‘cis-2-hexene’, ‘trans-2-hexene’, and ‘2-hexene’ (a mixture with unspecified cis/trans composition) are all assigned separate CAS Numbers.