A theme engine is a theme or plugin that changes the way WordPress renders themes. The difference is that a theme engine controls how the page is loaded, not how it looks.

While there are about seven dozen accepted definitions for “theme framework”, we decided that there is a clear difference between the two. Theme engines are a level below themes entirely. All themes run on a theme engine, framework or not.

WordPress’ default theme engine consists of the template hierarchy and a handful of other files like header.php, sidebar.php, and comments.php.

For instance, you could write a theme engine that redirects every single page to the wikipedia article on Cabbage, but it wouldn’t be of much use. The Elastic theme engine is focused on creating module-based themes instead of page-based themes.