Source : Free On-Line Dictionary of Computing

negative acknowledgement
     
        1.  (NAK) The {mnemonic} for {ASCII} character 21.
     
        Sometimes used as the response to receipt of a corrupted
        {packet} of information.
     
        Opposite of {acknowledgement}.
     
        2.  (NAK) Any message transmitted to indicate
        that some data has been received incorrectly, for example it
        may have a {checksum} or message length error.  A NAK message
        allows the sender to distinguish a message which has been
        received in a corrupted state from one which is not received
        at all.
     
        An alternative is to use only {ACK} messages, in which case
        the non-receipt of an ACK after a certain time is counted as a
        NAK but gives no information about the integrity of the
        communications channel.
     
        See also {ACK}.
     
        (1997-01-07)