Negative acknowledge character

"Nak" and "NAK" redirect here. For other uses, see Nak (disambiguation).

In telecommunications, a negative-acknowledge character (NAK or NACK) is a transmission control character sent by a station as a negative response to the station with which the connection has been set up.
In Binary Synchronous Communications 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.
In multipoint systems, the NAK is used as the not-ready reply to a poll.
In the ASCII code, the NAK character is 0x15 (hexadecimal).