Definition of "palladium"

The trial by jury is the Palladium of our civil rights.

1765–1769, William Blackstone, Commentaries on the Laws of England, (please specify |book=I to IV), Oxford, Oxfordshire: […] Clarendon Press

[H]ow unspeakably ominous to dim Royalist participators; for whom Royalism was Mankind's palladium[.]

1837, Thomas Carlyle, The French Revolution: A History […], volumes (please specify |volume=I to III), London: Chapman and Hall, (please specify the book or page number)

The presupposition of the Prometheus myth is to be found in the extravagant value which a naive humanity attached to fire as the true palladium of every ascending culture.

1967, Friedrich Nietzsche, translated by Walter Kaufman, The Birth of Tragedy, published 1872, in 1967