The Collaborative International Dictionary of English v.0.48:

Diaphragm \Di"a*phragm\, n. [L. diaphragma, Gr. ?, fr. ? to
   fence by a partition wall; dia` through + ?, ?, to fence,
   inclose; prob. akin to L. fareire to stuff: cf. F.
   diaphragme. See Farce.]
   1. A dividing membrane or thin partition, commonly with an
      opening through it.
      [1913 Webster]

   2. (Anat.) The muscular and tendinous partition separating
      the cavity of the chest from that of the abdomen; the
      midriff.
      [1913 Webster]

   3. (Zool.) A calcareous plate which divides the cavity of
      certain shells into two parts.
      [1913 Webster]

   4. (Opt.) A plate with an opening, which is generally
      circular, used in instruments to cut off marginal portions
      of a beam of light, as at the focus of a telescope.
      [1913 Webster]

   5. (Mach.) A partition in any compartment, for various
      purposes.
      [1913 Webster]

   Diaphragm pump, one in which a flexible diaphragm takes the
      place of a piston.
      [1913 Webster]

WordNet (r) 3.0 (2006):

diaphragm
    n 1: a mechanical device in a camera that controls size of
         aperture of the lens; "the new cameras adjust the diaphragm
         automatically" [syn: diaphragm, stop]
    2: (anatomy) a muscular partition separating the abdominal and
       thoracic cavities; functions in respiration [syn:
       diaphragm, midriff]
    3: a contraceptive device consisting of a flexible dome-shaped
       cup made of rubber or plastic; it is filled with spermicide
       and fitted over the uterine cervix [syn: diaphragm,
       pessary, contraceptive diaphragm]
    4: electro-acoustic transducer that vibrates to receive or
       produce sound waves

The Devil's Dictionary (1881-1906):

DIAPHRAGM, n.  A muscular partition separating disorders of the chest
from disorders of the bowels.