A noblewoman is a woman in the highest social class who has a title — duchess, countess, baroness, and names like that. Same world as nobleman, other half of the pair. Formal, mostly history and stories, or talk about real titled families. Though actually, English used to write as if man covered everyone; this word is there because the title and the life were a woman’s too. Again, noble is high-born here, not “a good woman”. A rich woman with no title is not a noblewoman.