The Free On-line Dictionary of Computing (30 December 2018):

Service Access Point

    (SAP) The OSI term for the component of a
   network address which identifies the individual application on
   a host which is sending or receiving a packet.

   TCP/IP's equivalent term is "port".

   Different SAPs distinguish between different services or
   applications on a host, e.g. electronic mail, FTP, HTTP.

   (1996-12-23)