校验码通常是一组数字的最后一位，由前面的数字通过某种运算得出，用以检验该组数字的正确性。常见的校验码有中华人民共和国居民身份证的最后一位，ISBN号码的最后一位等。

不同的校验码的算法常常不同。

A check digit is a form of redundancy check used for error detection on identification numbers (e.g. bank account numbers) which have been input manually. It is analogous to a binary parity bit used to check for errors in computer-generated data. It consists of a single digit (sometimes more than one) computed by an algorithm from the other digits (or letters) in the sequence input.