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diaphragm (n.)

late 14c., diafragma, in anatomy, "muscular membrane which separates the thorax from the abdominal cavity in mammals," from Late Latin diaphragma, from Greek diaphragma "partition, barrier, muscle which divides the thorax from the abdomen," from diaphrassein "to barricade," from dia "across" (see dia-) + phrassein "to fence or hedge in," which is of uncertain etymology. Beekes suggests it is a substrate word and finds "no convincing correspondence outside Greek."

The native word in the anatomical sense is midriff. From 1650s as "a partition" of any kind, "something which divides or separates;" 1660s in the special sense "thin piece of metal" serving some purpose (as a sound resonator, etc.). Meaning "contraceptive cap" is from 1933. Related: Diaphragmal; diaphragmatic.

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Definitions of diaphragm from WordNet

diaphragm (n.)
a mechanical device in a camera that controls size of aperture of the lens;
the new cameras adjust the diaphragm automatically
Synonyms: stop
diaphragm (n.)
(anatomy) a muscular partition separating the abdominal and thoracic cavities; functions in respiration;
Synonyms: midriff
diaphragm (n.)
a contraceptive device consisting of a flexible dome-shaped cup made of rubber or plastic; it is filled with spermicide and fitted over the uterine cervix;
Synonyms: pessary / contraceptive diaphragm
diaphragm (n.)
electro-acoustic transducer that vibrates to receive or produce sound waves;
From wordnet.princeton.edu