U+0015 "NAK" NEGATIVE ACKNOWLEDGE Unicode Character

U+0015 "NAK" NEGATIVE ACKNOWLEDGE is a control character originally defined in the ASCII standard as part of the C0 control code set, used in data transmission protocols to signal that a receiver has detected an error in a previously sent block of data and requests a retransmission. Its function is the opposite of the positive acknowledgment character (ACK), indicating that the transmitted information was not received correctly or was corrupted. While rarely used in modern user-facing text, this character remains a foundational element in legacy telecommunications systems, such as early binary synchronous communication protocols, where it helped maintain data integrity over noisy lines. As a control code, U+0015 does not represent a visible glyph or symbol, but it carries operational significance in low-level data exchange and error handling routines.