the breastpiece: the ornamented cloth pouch worn on the chest by Israel's high priest, set with twelve engraved stones and holding the objects used to seek God's decisions.
This word names one specific object, described in close detail when Israel's worship is set up in Exodus. It was a square of richly woven fabric, folded to form a pouch, that the high priest wore over his chest. Fixed to its front were twelve precious stones, each engraved with the name of one of the tribes of Israel, so that when the priest went before God he carried the whole nation over his heart.
The pouch also held the Urim and Thummim, the objects Israel used to seek a yes-or-no answer from God. That is why the Bible calls it the breastpiece of decision, or of judgment: it was the place where the priest represented the people before God and where God's verdict was sought. It was an instrument of worship and guidance, not a piece of jewelry.
Because it always points to this one priestly object, the word carries a single meaning throughout, spread across the instructions for making the high priest's clothing and the account of those clothes actually being made and put on.