<web> The default HTML page served by most web servers in response to a request for a directory. The name suggests that the page will contain some kind of index of the contents of the requested directory.
For example, if the content for website example.com is stored in the file system in directory /var/www/example.com, then a request for http://example.com/products would return the contents of file /var/www/example.com/products/index.html.
It is often possible, and occasionally necessary, to specify index.html explicitly in the URL, as in http://example.com/index.html, though modern practice is to omit it.
If you're looking for FOLDOC's home page (//foldoc.org/) at http://foldoc.org/index.html, then you followed an out-of-date link. Please update your bookmark to http://foldoc.org/ or inform the owner of the site you came from.
Microsoft, of course, has to be different and uses default.htm instead of index.html. The variant index.htm is a throw-back to the days when some file systems only allowed three-character file name extensions.