I don't think there is a formal name for such keys. A useful name would depend on context as much as anything else. For example, if the containing hash comprised a set of jobs then job key would be appropriate. If the containing hash contained a set of game nodes then room key might be appropriate. For a generic hash I'd tend to use primary key. The bottom line is, do what makes sense in your context to convey the intended usage to anyone reading your code.

I just call them "keys" and call the keys in the subordinate hashes "subkeys" recursively, so I may have keys, subkeys, and subsubkeys.

First-tier keys is an idea; in the spirit of a good (non-Soviet) poll, I'd like to nominate "master key".