Source : Free On-Line Dictionary of Computing

ephemeral port
     
         A {TCP} or {UDP} {port} number that is
        automatically allocated from a predefined range by the {TCP/IP
        stack} software, typically to provide the port for the client
        end of a {client-server} communication.
     
        {BSD} used ports 1024 through 4999 as ephemeral ports, though
        it is often desirable to increase this allocation.
     
        {(http://www.ncftpd.com/ncftpd/doc/misc/ephemeral_ports.html)}.
     
        (2002-10-06)