Check digit
The last digit of a barcode or ID number, computed from the rest, that catches most misreads and typos.
The digit is derived from the preceding ones so that an error in keying or scanning no longer adds up arithmetically. UPC and EAN use a weighted mod-10 (alternating times one and times three), ISBN-10 used mod-11 (with X standing for the value ten), and the Luhn algorithm guards card numbers.
It catches every single wrong digit and most swaps of two adjacent ones — the two commonest human mistakes. It is not a security measure but a typo detector, trivial to forge. The reader recomputes the digit and rejects the read if it disagrees.