plastron

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A metal breastplate worn under a coat of mail.
• noun A quilted pad worn by fencers to protect the torso and side.
• noun A trimming on the front of a bodice.
• noun The front of a man's dress shirt.
• noun The front panel of the tunic of a uniform, usually of a different color than the rest.
• noun Zoology The ventral part of the shell of a turtle or tortoise.

from The Century Dictionary.

• noun In the Echinodermata, a space surrounded by the subanal fasciole lying beneath the anus, in spatangoid echinoids.
• noun A breastplate; a garment or part of a garment covering the breast. , , ,
• noun In herpetology: The ventral part of the shell of a chelonian or testudinate; the lower shell, or under side of the shell, of a turtle or tortoise: more or less opposed to carapace.
• noun One of the similar exoskeletal plates developed upon the under side of the body of certain Amphibia, as the Labyrinthodonta.
• noun In mammalogy, the ventral shield or cuirass of the glyptodons or fossil armadillos.
• noun In anatomy, the sternum with the costal cartilages attached, as removed in autopsies.
• noun In ornithology, a colored area on the breast or belly of a bird, like or likened to a shield.

from the GNU version of the Collaborative International Dictionary of English.

• noun A piece of leather stuffed or padded, worn by fencers to protect the breast.
• noun (Anc. Armor) An iron breastplate, worn under the hauberk.
• noun (Anat.) The ventral shield or shell of tortoises and turtles. See Testudinata.
• noun A trimming for the front of a woman's dress, made of a different material, and narrowing from the shoulders to the waist.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The nearly flat part of the shell structure of a tortoise or other animal, similar in composition to the carapace.
• noun fencing A half-jacket worn under the jacket for padding or for safety.
• noun An ornamental front panel on a woman's bodice.
• noun A film of air trapped by specialized hairs against the body of an aquatic insect, and which acts as an external gill.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a metal breastplate that was worn under a coat of mail
• noun a large pad worn by a fencer to protect the chest
• noun the ornamental front of a woman's bodice or shirt
• noun the front of man's dress shirt
• noun (zoology) the part of a turtle's shell forming its underside