From WordNet (r) 3.0 (2006) [wn]:
chateau
n 1: an impressive country house (or castle) in France
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]