CHECKDIGIT
Check digit
A check digit is a form of redundancy check used for error detection on identification numbers which have been input manually. It is thus analogous to a binary parity bit used to check for errors in computer-generated data. It consists of a single digit computed by an algorithm from the other digits in the sequence input.The above text is a snippet from Wikipedia: Check digit
and as such is available under the Creative Commons Attribution/Share-Alike License.