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

server-side include

    (SSI) The facility provided by most web
   servers, e.g. NCSA httpd, to replace special tags in an
   HTML file with the contents of another file before the file
   is sent out by the server, i.e. an HTML macro.

   NCSA httpd tutorial
   (http://hoohoo.ncsa.uiuc.edu/docs/tutorials/includes.html).

   (1996-09-08)