The Collaborative International Dictionary of English v.0.48:

Sioux \Sioux\, prop. n. sing. & pl. (Ethnol.)
   A nation of American Indians; see Dakotas.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

Sioux
    n 1: 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: Sioux, Siouan]