Diaphragm  n. 1 muscular partition between the thorax and abdomen in mammals. 2 = *dutch cap. 3 a photog. Plate or disc pierced with a circular hole to cut off marginal beams of light. B vibrating disc in a microphone, telephone, loudspeaker, etc. 4 device for varying the lens aperture in a camera etc. 5 thin sheet as a partition etc. [greek phragma fence]