The Collaborative International Dictionary of English v.0.48:

Mitten \Mit"ten\, n. [OE. mitaine, meteyn, F. mitaine, perh. of
   Celtic origin; cf. Ir. miotog, Gael. miotag, Ir. & Gael.
   mutan a muff, a thick glove. Cf. Mitt.]
   1. A covering for the hand, worn to defend it from cold or
      injury. It differs from a glove in not having a separate
      sheath for each finger. --Chaucer.
      [1913 Webster]

   2. A cover for the wrist and forearm.
      [1913 Webster]

   To give the mitten to, to dismiss as a lover; to reject the
      suit of. [Colloq.]

   To handle without mittens, to treat roughly; to handle
      without gloves. [Colloq.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

mitten
    n 1: glove that encases the thumb separately and the other four
         fingers together