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

Moby Thesaurus II by Grady Ward, 1.0:

128 Moby Thesaurus words for "diaphragm":
   IUD, abdomen, abomasum, acoustical network, bag, barrier,
   bay window, beerbelly, belly, birth control device, brattice,
   breadbasket, buffer, buffer state, bulkhead, bumper,
   capacitor speaker, center, cloison, coaxial speaker, collision mat,
   condom, cone, contraceptive, contraceptive foam, core, craw, crop,
   crossover network, cushion, diameter, dissepiment, dividing line,
   dividing wall, division, dynamic speaker, earphone,
   electrodynamic speaker, electromagnetic speaker,
   electrostatic speaker, embonpoint, equator, excited-field speaker,
   fender, first stomach, full-fidelity speaker, gizzard, gullet, gut,
   headphone, headset, heart, high-fidelity speaker,
   high-frequency speaker, honeycomb stomach, horn, interior,
   interseptum, intrauterine device, kernel, kishkes, loudspeaker,
   low-frequency speaker, manyplies, mat, maw, mean, median, middle,
   midmost, midrange speaker, midriff, midsection, midst,
   monorange speaker, moving-coil speaker, nucleus, omasum,
   oral contraceptive, pad, panel, paries, partition, party wall,
   paunch, permanent magnet speaker, pessary, pot, potbelly, potgut,
   property line, prophylactic, psalterium, pusgut, rennet bag,
   reticulum, rubber, rumen, second stomach, separation, septulum,
   septum, shock pad, skin, spare tire, speaker, speaker system,
   speaker unit, spermicidal jelly, spermicide, stomach, swagbelly,
   the pill, thick, thick of things, third stomach, triaxial speaker,
   tum-tum, tummy, tweeter, underbelly, ventripotence, voice coil,
   waist, waistline, wall, woofer, zone



The Devil's Dictionary (1881-1906):

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