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. ICations> (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)