A flag in a TCP packet, short for "Synchronise", which is used to set up a connection.

To set up a TCP connection there are three packets involved, the initiator sends "SYN" to the reciever, the reciever acks the SYN with a packet with the "SYN" and "ACK" flags set, the initiator then ACK's the recievers SYN ACK packet.

See TcpStates