Source : Webster's Revised Unabridged Dictionary (1913)

Sioux \Sioux\, n. sing. & pl. (Ethnol.)
   See {Dakotas}.

Dakotas \Da*ko"tas\, n. pl.; sing. {Dacota}. (Ethnol.)
   An extensive race or stock of Indians, including many tribes,
   mostly dwelling west of the Mississippi River; -- also, in
   part, called {Sioux}. [Written also {Dacotahs}.]

Source : WordNet®

Sioux
     n : a member of a group of North American Indian peoples who
         spoke a Siouan language and who ranged from Lake Michigan
         to the Rocky Mountains [syn: {Siouan}]