The Collaborative International Dictionary of English v.0.48:

Peeress \Peer"ess\, n.
   The wife of a peer; a woman ennobled in her own right, or by
   right of marriage.
   [1913 Webster] Peerie

WordNet (r) 3.0 (2006):

peeress
    n 1: a woman of the peerage in Britain [syn: Lady,
         noblewoman, peeress] [ant: Lord, noble, nobleman]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

PEERESS. A noblewoman, the wife of a peer.