In some digital communication protocols, ACK is the name of a signal that data has been received successfully (for example, with an acceptable number of errors).
The ACK signal is sent by the receiving station (destination) back to the sending station (source) after the receipt of a recognizable block of data of specific size. In order to be recognizable, the data block must conform to the protocol in use. When the source receives the ACK signal from the destination, it transmits the next block of data. If the source fails to receive the ACK signal, it either repeats the block of data or else ceases transmission, depending on the protocol.
The ACK signal is usually an ASCII character that is reserved for that purpose. In some protocols, there are various ACK signals that indicate the successful reception and recognition of specific commands, such as power-down or standby.