ISBN
Also called: International Standard Book Number
A 13-digit identifier for a specific edition and format of a book, assigned per product rather than per title.
ISBNs identify products, not works. The hardback, paperback, and ebook of the same title each have their own, and a new edition gets a new one. That is precisely why an ISBN is useful in a reference: it pins down which physical thing you read and therefore which pagination your page numbers refer to.
The last digit is a checksum, so a mistyped ISBN can be detected rather than silently pointing at nothing. The older 10-digit form is still in circulation and converts to the 13-digit form by prefixing 978 and recomputing the check digit.
Most citation styles do not require an ISBN in the reference entry — it is more useful for looking a book up than for citing it. Handing one to a citation generator retrieves the title, authors, publisher, and year.