https://en.wikipedia.org/wiki/Top-level_domain - TLD is first level[1]. Domain directly on root. Domains like co.uk and so on are second level domains.
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[1] - or second, then again counting root as level 0 seems reasonable.
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[1] - or second, then again counting root as level 0 seems reasonable.
So there's an awkward way of defining things in various contexts, like cookie permissions, whether a domain is registerable, whether it is a registry the next level up
As the original comment suggested it's a bit of a non sequitur to call anything below the initial host label to be a TLD. I was just suggesting there isn't a generally accepted moniker for what they're called.
The public suffix list makes some kind of definition, mainly for cookie-level permissions.