\begin{definition}[Definition:Check Digit]
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.
\end{definition}