An ISSN is an identification number for serial publications including journals, magazines, and newspapers.
Here is the definition from the National Library of Canada: The International Standard Serial Number (ISSN) is an internationally recognized identification number for serial publications. An ISSN is an eight-digit standard number that, once assigned, becomes a permanent attribute of the individual serial for as long as it is issued under a given title. Each time the title of a serial is changed, a new ISSN must be assigned. ISSN must never be reused for new or changed serial titles.
ISSN are assigned by national and regional centres of the international ISSN Network. This system of international coordination ensures that each ISSN is unique to one serial publication.