isr.co.ro : glossary Acknowledge (ACK) A character or signal indicating successful accomplishment of a task. This message is used in data transmission to indicate that a block of data has been successfully received or that a device is ready to receive more data. In case of error, the NAK (negative acknowledgment) character or signal is sent instead. More specifically, the standard ASCII control character with a numerical value of six frequently used to indicate this message. It is also written as "^F"(control-F).