International Standard Serial Number
The International Standard Serial Number (ISSN) is a unique eight-digit serial number which identifies a serial publication of a specific title. (although the "numbers" can end with the letter X). Books have an ISBN.
As a unique identification number, ISBNs are handy to have when searching for titles which may have changed or be used by more than one journal. Many catalogs and subscription vendors use the ISSN for this purpose.