The Collaborative International Dictionary of English v.0.48:

Mixer \Mix"er\, n.
   1. One who, or that which, mixes.
      [1913 Webster]

   2. A person who has social intercourse with others of many
      sorts; a person viewed as to his casual sociability; --
      commonly used with some characterizing adjective; as, a
      good mixer; a bad mixer. [Colloq. or Slang, U. S.]
      [Webster 1913 Suppl.]

   3. a social gathering, game, or dance organized to provide an
      opportunity for people to meet each other; as, on the
      first night of the conference they had a wine-and-cheese
      mixer.
      [PJC]

   4. a nonalcoholic beverage (such as fruit juice, club soda or
      ginger ale) added to an alcoholic beverage to produce a
      mixed drink.
      [PJC]

   5. any device used for mixing.
      [PJC]

   6. an electronic device for blending or manipulating sounds
      from different sources to produce a composite soundtrack,
      for an audio recording, video recording, or a movie.
      [PJC]

   7. The technician who operates a mixer[6].
      [PJC]

WordNet (r) 3.0 (2006):

mixer
    n 1: a party of people assembled to promote sociability and
         communal activity [syn: sociable, social, mixer]
    2: club soda or fruit juice used to mix with alcohol
    3: electronic equipment that mixes two or more input signals to
       give a single output signal
    4: a kitchen utensil that is used for mixing foods