International Standard Serial Number
Appearance
An International Standard Serial Number (ISSN) is an eight-digit code to uniquely identify a periodical publication (periodical), such as a magazine. The ISSN is especially helpful in distinguishing between serials with the same title. ISSNs are used in ordering, cataloging, interlibrary loans, and other practices in connection with serial literature.
CERN series, such as the Yellow reports, have an ISSN.
For more information, see Wikipedia.