Breastplate is armor worn in battle. Moroni prepared the people of Nephi “with breastplates and with arm-shields, yea, and also shields to defend their heads” (Alma 43:19). “They have brought breastplates, which are large, and they are of brass and of copper, and are perfectly sound” (Mosiah 8:10).
Moroni “fastened on his head-plate, and his breastplate, and his shields” before he took the pole bearing his rent coat (Alma 46:13). The Nephites’ vital parts were shielded “by their breastplates, and their armshields, and their head-plates” from Lamanite strokes (Alma 43:38).
The Lamanites “had also prepared themselves with shields, and with breastplates” and thick garments of skins (Alma 49:6). A speaker said, “it is your breastplates and your shields that have preserved you” from swords (Alma 44:9).