By default our servers will look for (in order) a file by the name of index.html, index.cgi, index.php, index.htm, default.htm or home.htm in a given directory and will load the first file it finds as the directory's (or site's) index or "homepage". If you wish to use an alternate file name as a default index page, this can be done by adding the following directive to a .htaccess file stored either in your public_html directory (if you wish the directive to be in effect for your your entire site) or in a specific folder:
For example if you wish the default index page in a given directory to be "default.php", you would place a .htaccess file in that folder containing the following line:
Things to Note
You can specify multiple filenames and the server will look for them in the order you specify and set the first valid filename found as the directory's index page. If you intend on setting a non-standard index page as your site's main index page you'll likely want to include more standard index filenames along with your custom name so that you will be able to use standard index files such as index.html or index.php in subdirectories without having to add separate .htaccess files.
DirectoryIndex page1.html index.html index.php index.htm
With the directive above in place the server will first look for a file named "page1.html" and if found, will load it as the directory index. If no file by that name exists in a given directory, it will look for (in order) files named index.html, index.php and index.htm and load the first file it finds as the index page.