index

A file or directory on a server that usually contains information about the directory or Web site, such as access privileges, dates, and even a list of other indexes (indices).

A file called "index.html" is most likely the starting point (or homepage) for a Web site; a server will most likely be configured to automatically display the index.html (or .htm) file when a request for a Web site comes to the server. This means it is the default page that appears when you type in a URL, for example, sites that use PHP may have index.php as the default page.