The Collaborative International Dictionary of English v.0.48:

Noblewoman \No"ble*wom`an\, n.; pl. Noblewomen.
   A female of noble rank; a peeress.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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