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.

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

3. (Zoöl.) A calcareous plate which divides the cavity of certain shells into two parts.

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.

5. (Mach.) A partition in any compartment, for various purposes.

• (anatomy) In mammals, a sheet of muscle separating the thorax from the abdomen, contracted and relaxed in respiration to draw air into and expel air from the lungs.
  
• (mechanics) A flexible membrane separating two chambers and fixed around its periphery that distends into one or other chamber depending on the as the difference in the pressure in the chambers varies.