peeress

A noblewoman married to a peer.

But there were more than a score of peers there beside, with their peeresses in tall feathers, diamonds, and monstrous hoops.

A woman holding a noble title in her own right.

[I]t is obvious that the Californian heiress is not the only gainer by an alliance which will enable her to make the easy and common transition from a Republican lady to a British peeress.