palladium

Something believed to ensure protection or safety; a safeguard. Also: a chemical element (Pd, atomic number 46).

• (no common derivatives in the figurative sense)

• palladium of liberty
• palladium of freedom
• palladium against tyranny

1. 1.The Constitution was designed as the palladium of American liberty — the ultimate safeguard against tyranny. (protector)
2. 2.A free press has long been considered the palladium of democratic governance. (essential safeguard)

• safeguard/ˈseɪfɡɑːrd/— a measure of protection
• bulwark/ˈbʊlwərk/— a defensive wall or protection
• shield/ʃiːld/— something that protects

from Greek "Palladion" — a sacred statue of Pallas Athena in Troy, believed to protect the city. Troy only fell after Odysseus stole the Palladium.