What is protocol and host combined called?

I think the term you may really be looking for is origin, as in RFC 6454.

An origin isn’t just scheme+host, but scheme+host+port, and URLs like https://example.com aren’t actually just scheme+host — because they express a (default) port too: 443, the default TLS port — just as in http://example.com there’s a built-in expression that the port is 80.

I’m not sure what the context was for the original question here, but I know a common case that would normally motivate somebody to want to know what a combination of scheme+host is called is the case where you want to compare two URLs that either have the same host but different schemes, like https://example.com and http://example.com, or that have the same scheme and host but different ports, like http://example.com and http://example.com:8888. And consideration of comparing URLs that way will eventually take you to thinking of them in terms of the “same origin” policy that the entire Web security model is based on.

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