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

It is called the origin.

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.