Breastplate \Breast"plate`\, n.

1. A plate of metal covering the breast as defensive armor.

Before his old rusty breastplate could be scoured, and his cracked headpiece mended. --Swift.

2. A piece against which the workman presses his breast in operating a breast drill, or other similar tool.

3. A strap that runs across a horse's breast. --Ash.

4. (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.

Source: Webster's Revised Unabridged Dictionary (1913)