Definition:Check Digit

Definition
A check digit is a digit or other symbol used as an aid to identifying whether a mistake has been made transmitting a codeword.

It is calculated from the symbols in the codeword itself, and appended to the codeword as an extra symbol.

If a mistake is made transmitting the codeword, the check digit will then be incompatible with the rest of the contents of the codeword, and reveal that a transmission error is present.