diaphragm

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   Any of various membranes or sheets of muscle or ligament which separate one cavity from another.
3. 3
   A contraceptive device consisting of a flexible cup, used to cover the cervix during intercourse.
4. 4
   A flexible membrane separating two chambers and fixed around its periphery that distends into one or other chamber as the difference in the pressure in the chambers varies.
5. 5
   In a speaker, the thin, semi-rigid membrane which vibrates to produce sound.
6. 6
   A thin opaque structure with a central aperture, used to limit the passage of light into a camera or similar device.
7. 7
   A permeable or semipermeable membrane.
8. 8
   A floor slab, metal wall panel, roof panel or the like, having a sufficiently large in-plane shear stiffness and sufficient strength to transmit horizontal forces to resisting systems.

Verb

1. 1
   To reduce lens aperture using an optical diaphragm.
2. 2
   To act as a diaphragm, for example by vibrating.