1. An acronym for SUMmation CHECK. In data communications, an error-checking technique in which the number of bits in a unit of data is summed, transmitted along with the data, and checked by the receiving computer. If the sum differs, an error probably occurred in transmission and the transmission is repeated. A commonly used personal computer communications protocol called XMODEM uses the checksum technique. In some virus scanning and file integrity software checksums are calculted for every file in a directory and the results are stoed in the directory. When the program is scanningm it compares the checksum information stored in the directory with the current checksum for each scanned file. A difference in the sum may indicate that the file has been infected by a virus that doesn't leave a recognised signature. From QUECID

2. A number computed by adding together all the characters from an entire file in a special mathematical way. It is useful for ensuring a file has been transferred correctly. From Linux Guide @FirstLinux

3. A technique for detecting if data inadvertently changes during transmission. The sender simply divides all the data up into two-character numbers, then adds all the numbers together. The receiver makes the same calculation, and checks the calculated checksum with the transmitted checksum. If they don't match, then the receiver knows the data was corrupted in transit. Key point: Checksums are not secure against intentional changes by hackers. For that, you need a cryptographic hash. From Hacking-Lexicon