V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

ISAPI
       Internet Server Application Programmer's Interface (MS, C/S, WWW,
API)


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

Internet Server Application Programming Interface
ISAPI

    (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 filters

   (1997-01-06)