Source : Free On-Line Dictionary of Computing

Internet Server Application Programming Interface
     
         (ISAPI) {Microsoft}'s programming interface
        between applications and their {Internet Server}.  Active
        Servers created with ISAPI extensions can be complete
        in-process applications themselves, or can "connect" to other
        services.  ISAPI is used for the same sort of functions as
        {CGI} but uses {Microsoft Windows} {dynamic link libraries}
        (DLL) for greater efficiency.  The server loads the DLL the
        first time a request is received and the DLL then stays in
        memory, ready to service other requests until the server
        decides it is no longer needed.  This minimises the overhead
        associated with executing such applications many times.
     
        An HTTP server can unload ISAPI application DLLs to free
        memory or preload them to speed up the first access.
        Applications can also be enhanced by {ISAPI filter}s
     
        (1997-01-06)