Short for server-side include, a type of HTML comment that directs the Web server to dynamically generate data for the Web page whenever it is requested. SSIs can also be used to execute programs and insert the results. They therefore represent a powerful tool for Web developers. There is no official standard for SSIs, so every Web server is free to support different SSIs in different manners. However, many SSI commands, such as #include and #exec, have become de facto standards. Web pages that contain SSIs often end with a .shtml extension, though this is not a requirement. The filename extension enables the Web server to differentiate those pages that need to be processed before they are sent to the browser.
Meaning of SSI in English
DAX English glossary of key connectivity terms. Английский глоссарий основных терминов по коммуникациям DAX. 2012