Source : Free On-Line Dictionary of Computing

server-side include
     
         (SSI) The facility provided by several {HTTP}
        {servers}, e.g. {NCSA httpd}, to replace certain {HTML} {tag}s
        in one {HTML} file with the contents of another file at the
        time when 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)