Negative-acknowledge character
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In telecommunications, 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
See ACK