A digit representing the sum of the digits in an instance of digital data; used to check whether errors have occurred in transmission or storage WordNet 3.0 & Open English WordNet

A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. Wikipedia

A mathematical process that can be applied to a file or other data to create a unique number relative to the contents of that file. If the file is modified, the checksum will change, usually indicating that the file in question has failed to download correctly or has been modified in some way. OmegaWiki

A number, derived by applying a suitable algorithm to some data, used to check whether errors have occurred in transmission, storage or data entry. Wiktionary

Number serving to detect error. Wiktionary (translation)