the solemn, God-given charter of the covenant: the binding terms that also stand as a lasting witness to the bond between God and his people
This word names the written, authoritative record of God's covenant with Israel, the document that both lays down the terms of the relationship and stands as its permanent witness. Older translations call it "the testimony" because it testifies; many recent translations call it "the pact" because it binds. The two are not really rivals. In the ancient world a sworn agreement was made real by a written text that was kept in a sacred place, and that same text served as the standing proof of what each side had pledged. So this word holds both ideas at once: the charter that obligates, and the witness that attests.
Its most concrete use points to the two stone tablets of the Ten Commandments. They are "the testimony" placed inside the ark, and from that the ark, the tabernacle, the tent, the tablets, and the inner curtain are all called things "of the testimony," simply because they stand near or hold those tablets. The same chest that Exodus calls the ark of the testimony, Deuteronomy calls the ark of the covenant, which is the clearest sign from within the Bible itself that the word means something very close to the covenant document.
At the king's coronation the word names an object handed to the new ruler as a token of legitimate, God-given authority, though exactly what that object was is uncertain. In the Psalms the word broadens into one of several names for God's revealed instruction, the trustworthy commands he has laid upon his people. Across all of these the core holds steady: this is the authoritative word God has set down, which both governs the relationship and bears witness to it.