The Collaborative International Dictionary of English v.0.48:

Nobleman \No"ble*man\, n.; pl. Noblemen.
   One of the nobility; a noble; a peer; one who enjoys rank
   above a commoner, either by virtue of birth, by office, or by
   patent.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nobleman
    n 1: a titled peer of the realm [syn: Lord, noble,
         nobleman] [ant: Lady, noblewoman, peeress]

The Devil's Dictionary (1881-1906):

NOBLEMAN, n.  Nature's provision for wealthy American minds ambitious
to incur social distinction and suffer high life.