The term “breastplate” is used to refer to the following two things in the Bible.
An ornament covering the breast of the high priest, first mentioned in Exodus 25:7. It was made of embroidered cloth, set with four rows of precious stones, three in each row. On each stone was engraved the name of one of the twelve tribes (Exodus 28:15-29; 39:8-21). It was in size about ten inches square [25.4 centimeters]. The two upper corners were fastened to the ephod by blue ribbons. It was not to be “loosed from the ephod” (Exodus 28:28). The lower corners were fastened to the girdle of the priest.
As it reminded the priest of his representative character, it was called the memorial (28:29).