diaphragm, term used to describe any of several large muscles, found in humans and other mammals, which separate two adjacent regions of the body. The most commonly known muscle of this class is the thoraco-abdominal diaphragm. In humans, the thoraco-abdominal diaphragm acts as a partition between the cavity of the chest and that of the abdomen. The chief muscle used in respiration, it is relaxed and dome-shaped during exhalation. During inhalation it contracts, pulling downward, and with the combined contraction of the chest muscles allows the chest cavity to expand. Any interference with its free movement, as in the paralysis of poliomyelitis, seriously impedes the function of the lungs and therefore endangers life. In its downward movement the thoraco-abdominal diaphragm also stimulates the stomach and liver and thus aids in the digestive processes. Spasmodic contraction results in hiccups. The thoraco-abdominal diaphragm is also subject to developmental defects, hernia, injury, displacement, and infection. Other diaphragms in the human body include the pelvic diaphragm and the urogenital diaphragm, which use similar muscular contractions and expansions in their respective functions.

Dome-shaped muscular and membranous structure between the thoracic and abdominal cavities. The principal muscle used in respiration, it is also important in coughing, vomiting, excretion, and other expulsive functions. Spasms of the diaphragm produce hiccups. The aorta passes behind the diaphragm; the inferior vena cava and esophagus pass through it. Protrusion of part of the stomach above the diaphragm is called a hiatal hernia.

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