1. (NAK) The mnemonic fORASCII 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 checksumOR 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)