Processing or content generation that is done on the web server or other server, as opposed to on the client computer where the web browser is running.
An example is server-side include where one file is inserted in another before it is served, rather than, say, having the browser request the files separately and combine them using an iframe. A very common kind of server-side processing is the inclusion of data from a database in a web page.
There are many software environments and technologies designed for server-side processing, e.g. CGI, ISAPI, WebObjects and ASP.