Stands for acknowledgment; any transmission from a receiving station to a transmitting station communicating that the transmitted data has been received without errors.
On the other hand, if the receiving station determines that the data transmission is late or has not arrived, a NAK (negative acknowledgment) is generated to indicate to the transmitting station that the data should be sent again.
The Transmission Control Protocol (TCP) is a connection-oriented protocol that relies on acknowledgments for successful transmission of data. When a stream of TCP packets is being sent over the network, each packet contains an acknowledgment number indicating the sequence number of the next packet that the receiving station should expect to receive.
TCP can use an ACK to acknowledge a series of TCP packets that have been received, rather than just a single packet. A TCP packet sent as an acknowledgment has its ACK flag set to 1 to indicate that the acknowledgment numbers of the packets received are valid.