(1.) That piece of ancient armour that protected the breast.
This word is used figuratively in Eph. 6:14 and Isa. 59:17. (See
ARMOUR T0000315.)

(2.) An ornament covering the breast of the high priest, first
mentioned in Ex. 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 (Ex.
28:15-29; 39:8-21). It was in size about ten inches square. The
two upper corners were fastened to the ephod by blue ribbons. It
was not to be "loosed from the ephod" (Ex. 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). It was also called the breastplate
of judgment (28:15). (See PRIEST T0003001.)