a treasured possession: property set aside as personally one's own and prized above all the rest of what one owns
Behind this rare word stands an old legal and economic idea, shared across the ancient Near East. A ruler owned a whole kingdom, yet still kept a private store of wealth that was his alone, held apart from the public treasury and dearer to him than the rest. That is the literal picture: a treasure deliberately reserved for oneself.
Israel's Bible takes this concrete word and uses it in two settings that are really one idea seen from two angles. In Chronicles and Ecclesiastes it keeps its plain sense, the personal gold and silver a king has gathered for himself. In Exodus, Deuteronomy, the Psalms, and Malachi the same word becomes the most intimate name God gives his people: out of all the nations on earth, every one of which already belongs to him, God reserves Israel as his own personal treasure.
The two uses belong together. The covenant language is not warmth loosely attached to the people; it is the property word applied to them. Because everything on earth is already God's, his choosing one people as his private treasure becomes all the more pointed, not less. And the literal sense in the later books is what keeps the covenant promise from softening into mere affection: God prizes and guards this people the way a king guards the wealth that is his alone.