Source : WordNet®

checksum
     n : 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

Source : Free On-Line Dictionary of Computing

checksum
     
         A computed value which depends on
        the contents of a block of data and which is transmitted or
        stored along with the data in order to detect corruption of
        the data.  The receiving system recomputes the checksum based
        upon the received data and compares this value with the one
        sent with the data.  If the two values are the same, the
        receiver has some confidence that the data was received
        correctly.
     
        The checksum may be 8 bits (modulo 256 sum), 16, 32, or some
        other size.  It is computed by summing the bytes or words of
        the data block ignoring {overflow}.  The checksum may be
        negated so that the total of the data words plus the checksum
        is zero.
     
        {Internet} {packets} use a 32-bit checksum.
     
        See also {digital signature}, {cyclic redundancy check}.
     
        (1996-03-01)