Diaphragm /(?)/

Di·a·phragm

Diaphragm

n.
  1. A dividing membrane or thin partition, commonly with an opening through it.
  2. The muscular and tendinous partition separating the cavity of the chest from that of the abdomen; the midriff. (Anat.)
  3. A calcareous plate which divides the cavity of certain shells into two parts. (Zool.)
  4. 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. (Opt.)
  5. A partition in any compartment, for various purposes. (Mach.)

Phrases & Compounds
Diaphragm pump
one in which a flexible diaphragm takes the place of a piston.