What do you call the entire first part of a URL?

I don't know the name for when it has the scheme, but the hostname with the port is collectively known as the Authority. A nice explanation here.

It is called the origin.

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More generally speaking, here are the different parts of a URL, as per Location. (So at least according to how Javascript calls it)

protocol://username:password@hostname:port/pathname?search#hash
-----------------------------href------------------------------
                             -----host----
-----------      origin      -------------

• protocol - protocol scheme of the URL, including the final ':'
• hostname - domain name
• port - port number
• pathname - /pathname
• search - ?parameters
• hash - #fragment_identifier
• username - username specified before the domain name
• password - password specified before the domain name
• href - the entire URL
• origin - protocol://hostname:port
• host - hostname:port

Note that the exact naming of each part may be different in different standards. For example, 'host' in RFC 6454 section 4. means 'hostname' in the above diagram.