The Collaborative International Dictionary of English v.0.48:

Chateau \Cha`teau"\, n.; pl. Chateux. [F. ch[^a]teau a castle.
   See Castle.]
   1. A castle or a fortress in France.
      [1913 Webster]

   2. A manor house or residence of the lord of the manor; a
      gentleman's country seat; also, particularly, a royal
      residence; as, the chateau of the Louvre; the chateau of
      the Luxembourg.
      [1913 Webster]

   Note: The distinctive, French term for a fortified castle of
         the middle ages is ch[^a]teau-fort.
         [1913 Webster]

WordNet (r) 3.0 (2006):

chateau
    n 1: an impressive country house (or castle) in France