The default page for a website or a directory within a website. The name derives from the fact that, for some systems, a link to a directory will default to a listing (index) of the directory’s contents.
http://mydomain.com/ may display http://mydomain.com/index.html and
http://mydomain.com/mydirectory/ may display http://mydomain.com/mydirectory/index.html
Depending upon the server setup other files such as index.htm, index.php or default.html may serve this function.