CAS Registry Numbers are unique numerical identifiers assigned by the Chemical Abstracts Service to every chemical described in the open scientific literature (currently including those described from at least 1957 through the present) and including elements, isotopes, organic and inorganic compounds, ions, organometallics, metals, nonstructurable materials (aka 'UVCB's- i.e., materials of Unknown, Variable Composition, or Biological origin). They are also referred to as CAS RNs, CAS Numbers, etc.

The Registry maintained by CAS is an authoritative collection of disclosed chemical substance information. Currently the CAS Registry identifies more than 56 million organic and inorganic substances and 62 million sequences, plus additional information about each substance; and the Registry is updated with an approximate 12,000 additional new substances daily.

Historically, chemicals have been identified by a wide variety of synonyms. Frequently these are arcane and constructed according to regional naming conventions relating to chemical formulae, structures or origins. Well-known chemicals may additionally be known via multiple generic, historical, commercial, and/or black-market names.