TCP half-open
A TCP connection is referred to as half-open when the station at one end of that TCP connection has crashed, or has otherwise removed the socket without formally notifying the other end of the TCP connection.
Due to the stateless nature of the Transmission Control Protocol, it is possible that if the application controlling the still-existing end of the connection is not expecting a response, then the socket may persist indefinitely in the state known as TCP half-open.
Note: A common misconception throughout the Internet is that the term TCP half-open applies to an partially completed TCP handshake state. A better term for this unsynchronized kind of socket state is an Embryonic connection state.