Negative-acknowledge character

In telecommunication, a negative-acknowledge character (NAK) is a transmission control character sent by a station as a negative response to the station with which the connection has been set up.

Note 1: In binary synchronous communication[?] protocol, the NAK is used to indicate that an error was detected in the previously received block and that the receiver is ready to accept[?] retransmission of that block.

Note 2: In multipoint systems, the NAK is used as the not-ready reply to a poll.

Source: from Federal Standard 1037C and from MIL-STD-188