peeress

plural peeresses

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, J[oseph] Sheridan Le Fanu, “Being a Chapter of Hoops, Feathers, and Brilliants, and Bucks and Fiddlers”, in The House by the Church-yard. […], volume II, London: Tinsley, Brothers, […], →OCLC, page 208

2. 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.

   1892 [April], A[rthur] Conan Doyle, “Adventures of Sherlock Holmes. X.—The Adventure of the Noble Bachelor.”, in Geo[rge] Newnes, editor, The Strand Magazine: An Illustrated Monthly, volume III (January to June), number [16], London: George Newnes, Limited, […], page 388, column 1