noun

• (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.
  "thoracic diaphragm"
• (anatomy) Any of various membranes or sheets of muscle or ligament which separate one cavity from another.
• (mechanics) 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.
  "There also were five broken panes of glass on the two huge, rubberlike diaphragms that equalize air pressure within the dome as the artificial atmosphere expands and contracts with temperature variations, he said."
• (acoustics) In a speaker, the thin, semi-rigid membrane which vibrates to produce sound.
  "acoustic diaphragm"

verb

• (optics, photography) To reduce lens aperture using an optical diaphragm.
  "He employs an equatorial with an object-glass having a focal length of five metres, and which was diaphragmed down to eight centimetres."