breastplate

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

• noun A piece of armor that covers the breast.
• noun Judaism A square cloth set with 12 precious stones representing the 12 tribes of Israel, worn over the breast by ancient high priests.

from The Century Dictionary.

• noun The iron plate used for covering the breast-hole of a cupola.
• noun A square ornament worn by the Jewish high priest, consisting of the same textile fabric as the ephod, and bearing twelve precious stones engraved with the names of the twelve tribes of Israel, set in the gold.
• noun The armor for the front of the body, when made in one piece reaching from the waist to about the collar-bone.
• noun A strap that runs across a horse's breast.
• noun A plate or piece which receives the buttend of a boring-tool, and is held against the breast when the tool is in use. Also called conscience and palette.
• noun The sternum or central piece on the lower side of the cephalothorax of a spider, between the bases of the legs.
• noun The lower shell or plastron of a tortoise.

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

• noun A plate of metal covering the breast as defensive armor.
• noun A piece against which the workman presses his breast in operating a breast drill, or other similar tool.
• noun A strap that runs across a horse's breast.
• noun (Jewish Antiq.) A part of the vestment of the high priest, worn upon the front of the ephod. It was a double piece of richly embroidered stuff, a span square, set with twelve precious stones, on which were engraved the names of the twelve tribes of Israel. See Ephod.

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

• noun A piece of armor that covers the chest.
• noun A piece of horse tack designed to prevent the saddle slipping backwards.

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

• noun armor plate that protects the chest; the front part of a cuirass