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.