Breastplate

a piece of plate armor partially or completely covering the front of the torso: used by itself or as part of a cuirass.
the part of the harness that runs across the chest of a saddle horse.
Judaism.

a square, richly embroidered vestment ornamented with 12 precious stones, each inscribed with the name of one of the 12 tribes of Israel, secured to the ephod of the high priest and worn on the chest. Ex. 28:15–28.
a rectangular ornament, typically of silver, suspended by a chain over the front of a scroll of the Torah.

a plate opposite the chuck end of a breast drill against which the operator’s chest is placed.
Historical Examples

The Dixie Book of Days Matthew Page Andrews
Micah Clarke Arthur Conan Doyle
A Friend of Caesar William Stearns Davis
His Masterpiece Emile Zola
Lippincott’s Magazine of Popular Literature and Science, Vol. 15, No. 90, June, 1875 Various
Alroy Benjamin Disraeli
The Cloister and the Hearth Charles Reade
The Children of Odin Padraic Colum
Spanish Arms and Armour Albert F. Calvert
Expositions of Holy Scripture Alexander Maclaren

noun
a piece of armour covering the chest
the strap of a harness covering a horse’s breast
(Judaism) an ornamental silver plate hung on the scrolls of the Torah
(Old Testament) a square vestment ornamented with 12 precious stones, representing the 12 tribes of Israel, worn by the high priest when praying before the holy of holies
(zoology) a nontechnical name for plastron