The Collaborative International Dictionary of English v.0.48:

Bellows \Bel"lows\, n. sing. & pl. [OE. bely, below, belly,
   bellows, AS. b[ae]lg, b[ae]lig, bag, bellows, belly. Bellows
   is prop. a pl. and the orig. sense is bag. See Belly.]
   An instrument, utensil, or machine, which, by alternate
   expansion and contraction, or by rise and fall of the top,
   draws in air through a valve and expels it through a tube for
   various purposes, as blowing fires, ventilating mines, or
   filling the pipes of an organ with wind.
   [1913 Webster]

   Bellows camera, in photography, a form of camera, which can
      be drawn out like an accordion or bellows.

   Hydrostatic bellows. See Hydrostatic.

   A pair of bellows, the ordinary household instrument for
      blowing fires, consisting of two nearly heart-shaped
      boards with handles, connected by leather, and having a
      valve and tube.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bellows
    n 1: a mechanical device that blows a strong current of air;
         used to make a fire burn more fiercely or to sound a
         musical instrument

Easton's 1897 Bible Dictionary:

Bellows
   occurs only in Jer. 6:29, in relation to the casting of metal.
   Probably they consisted of leather bags similar to those common
   in Egypt.