Definition: check digit
A numeric digit used to ensure that account numbers are entered accurately into the computer. Using a formula, a digit is calculated from each new account number, which is then made part of that number, either at the end, the beginning or somewhere in the middle of the number.
When an account number with the check digit is entered, the data entry program recalculates the check digit and compares it to the check digit entered. If the digits are not equal, the account number is considered invalid.