The Berkeley Unix mechansim for creating a virtual connection between processes. Sockets interface Unix's standard I/O with its network communication facilities. They can be of two types, stream (bi-directional) or datagram (fixed length destination-addressed messages). The socket library function socket() creates a communications end-point or socket and returns a file descriptor with which to access that socket. The socket has associated with it a socket address, consisting of a port number and the local host's network address.
Unix manual page: socket(2).