I don't inherently mean the home page, since some websites redirect (domain.com to domain.com/front for instance). I'm not referring to the domain name either, but the webpage.
To clarify, the page which matches the domain name, such as https://superuser.com/
The root page on your question can be index.html, index.php or index.other_extension, as well as many other things like banana.anything, depending on how we configure the webserver, or the programming language of the webpage.
For example, if you go to https://wordpress.org/index.php, it will redirect you back to https://wordpress.org/. It means this website use index.php as a default webpage.
In fact, we can change this page to anything we want, so there is no way to find out. The only way to find out is only take times to test. As you can see if you try https://superuser.com/index.php or index.html, there was no page exist.
What is the "root" page of a website called (e.g.
superuser.com) called?
Assuming there is an actual pre-existing page, index plus a file extension (e.g. index.html, index.php, etc.) is effectively the standard name for this page (i.e. what most web servers look for by default).
However, as already mentioned in the comments, this page doesn't always exist. This can occur when:
A domain is redirected (i.e. from HTTP to HTTPS or to another domain) or proxied internally.
A domain has no ex. index file and the server itself generates a given page (possibly including the "root" page of a domain).
A web server (such as Apache) allows the server administrator to change or remove index as a name for "default" return documents.
To clarify, the page which matches the domain name, such as
https://superuser.com/.
I am guessing you are aware of this, but these pages don't exist, at least as a standard (i.e. it's unlikely there is a ex. superuser.com.html, even though it could theoretically exist).
As near as I can tell, it is called the root page or root URL.
In a search for "root page" website, it comes up enough, and I didn't come across any alternative names for it.
“Root page” means the
mainweb page that appears when someone types in the root of your website’s domain. For example, https://yourwebsite.com would be a root web page request. (source)
Every web folder has a "default page" or "webserver directory index". (If no file is present or specified for the folder, the webserver might return a directory listing of that folder or a familiar error pagesuch as, "403 - Forbidden: Access is denied. You do not have permission to view this directory or page using the credentials that you supplied.", "404 - page not found", or "directory listing is not permitted".)
The root page of any website is the "webserver directory index" of its root folder.
If you visit https://wordpress.org/index.php, the URL in your browser changes to https://wordpress.org/. (the root page). https://wordpress.org/index.php is a redirect. https://wordpress.org/ is the root page and the homepage.
If you visit https://foo.bar/, the URL in your browser changes to https://foo.bar/index.php. https://foo.bar/ can still be called the root page, even though it is just a rewrite or a redirect. https://foo.bar/index.php is the home page, but I regard that as an unwise choice by the webmaster. It might have a benefit (a named page might cache better), but it definitely has penalties. I would always rather link and make shortcuts to the root page, https://foo.bar/, because that URL "has to work", and it cannot be renamed. (Well, they could change to https://www.foo.bar/, but that is a separate issue.) If I link to https://foo.bar/index.php, and then someone updates the website, all kinds of stupid things can happen. They should make https://foo.bar/index.php a redirect (or make 404s in the root folder redirect) to the right page, but I know I can't count on that -- I've seen dozens of broken homepage links. https://foo.bar/index.php could serve a blank page, a broken page, or (worst) a stale version of the homepage.
||youtube.com/$documentmatches the entire domain whereas||youtube.com/front/$documentwould block solely this specific page.https://example.com/index.html?