peeress

Etymology

peer + -ess

noun

1. 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. 1863, Sheridan Le Fanu, The House by the Churchyard
2. A woman holding a noble title in her own right.
   … it 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. 1892, Sir Arthur Conan Doyle, “The Noble Bachelor”, in The Adventures of Sherlock Holmes, HTML edition, The Gutenberg Project, published 2011