negative acknowledgement

negative acknowledgement [′neg·əd·iv ik′näl·ij·mənt]

(computer science)

In a data communications network, a control character returned from a receiving machine to a sending machine to indicate the presence of errors in the preceding block of data. Abbreviated NAK.

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1.	(character)	negative acknowledgement - (NAK) The mnemonic for ASCII character 21. Sometimes used as the response to receipt of a corrupted packet of information. Opposite of acknowledgement.
2.	(communications)	negative acknowledgement - (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.