International Standard Book Number. Developed in 1966 and adopted as an international standard in 1970, the ISBN is a unique, ten- or (since January 1, 2007) thirteen-digit number assigned to every published book. Hardcover and paperback editions of the same book are assigned different numbers, and new editions of books are assigned a new ISBN number.

Though it is usually pronounced as individual letters, some people do pronounce it as a word: iz-bun.