FIN - a flag set in a TCP packet indicating that one conversant in an current communication has finished sending data.

An ACK flag will (almost) always be piggybacked onto a TCP packet with a FIN set. A recipient of a FIN packet MUST acknowledge the receipt of the FIN by ACKing the received packet and incrementing the sequence number by one (SeqNum+1). Either side of the communicating pair can terminate their end of the communication first. The TCP State of the machine SENDING the FIN will change to FIN_WAIT1 once the FIN has been sent. Upon receipt of an ACK, the TCP State of the sending machine will change to FIN_WAIT2 (indicating that the communication is half-closed.) Upon receipt of a final FIN from the other conversant, the state will change to Closing. Finally, once the final FIN has been ACK'd, the state will change to Closed.

(See RFC:0793 - Transmission Control Protocol - for more information).