ISBN or International Standard Book Number is a unique 13-digit code assigned to every book published worldwide. However, there are various other terms that can be used synonymously for ISBN. Some of them include 'book identifier', 'ISBN number', 'book code', 'ISBN code', 'book number', and 'book registration number'. Additionally, there are also different variations of ISBN, such as the older 10-digit version, the EAN-13 barcode version, and the ISBN-13 version. Regardless of the term used to refer to it, the ISBN is an essential part of any book and is necessary for inventory management, marketing, and distribution purposes.