n.1. a muscular sheet that separates the thoracic and abdominal cavities. 2. a cup-shaped contraceptive device made from a layer of thick latex rubber fitted over a round or spiral spring. The diaphragm is filled with a contraceptive jelly and inserted in the vagina so that it forms a barrier between the cervix and any spermatozoa that enter the vagina during coitus. The spring holds it in place. The diaphragm has been used by women since 1882. —diaphragmaticadj.