The FCA Handbook Glossary defines an International Securities Identification Number (ISIN) as:
"a 12-character, alphanumeric code which uniquely identifies a financial instrument and provides for the uniform identification of securities at trading and settlement."
An ISIN uniquely identifies a security. Its structure is defined in ISO 6166. Securities for which ISINs are issued include bonds, commercial paper, stocks and warrants.
ISINs consist of two alphabetic characters, which are the ISO 3166-1 alpha-2 code for the issuing country, nine alpha-numeric digits (the National Securities Identifying Number, or NSIN, which identifies the security), and one numeric check digit. The NSIN is issued by a national numbering agency (NNA) for that country. Regional substitute NNAs have been allocated the task of functioning as NNAs in those countries where NNAs have not yet been established.
NNAs cooperate through the Association of National Numbering Agencies (ANNA). ANNA also functions as the ISO 6166 Registration Authority (RA).