palladium

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A soft, ductile, steel-white, tarnish-resistant, metallic element occurring naturally with platinum, especially in gold, nickel, and copper ores. Because it can absorb large amounts of hydrogen, it is used as a purification filter for hydrogen and a catalyst in hydrogenation. It is alloyed for use in electric contacts, jewelry, nonmagnetic watch parts, and surgical instruments. Atomic number 46; atomic weight 106.4; melting point 1,552°C; boiling point 3,140°C; specific gravity 12.02 (20°C); valence 2, 3, 4. See Table at element.
2. n. A safeguard, especially one viewed as a guarantee of the integrity of social institutions: the Bill of Rights, palladium of American civil liberties.
3. n. A sacred object that was believed to have the power to preserve a city or state possessing it.

Century Dictionary and Cyclopedia

1. n. A statue or image of the goddess Pallas; especially, in art and legend, a xoanon image. On the preservation of such an image, according to the legend, depended the safety of Troy.
2. n. 2. Anything believed or reputed to afford effectual defense, protection, and safety: as, trial by jury is the palladium of our civil rights.
3. n. Chemical symbol, Pd; atomic weight, 106.5. One of the rare metals associated with platinum. It was separated from native platinum by Wollaston in 1803, and named after the planet Pallas, which had just before that time been discovered by Olbers. Palladium is dimorphous. It occurs in Brazil native, in minute octahedral crystals; and on the Harz it has been found in small hexagonal plates. It is, however, a decidedly rare substance, and the chief supply comes from the working over of the platiniferous residues of various mints. It resembles platinum in appearance, but is harder; its specific gravity is 11.4. It fuses more readily than platinum or any other of the so-called platinum metals, melting, as is stated by some authorities, about as easily as wrought-iron. It is both ductile and malleable, and would be a very useful metal if it were not so scarce as to be expensive and irregularly attainable. The graduated surfaces of some astronomical instruments have been made of palladium, a use for which this metal is admirably adapted on account of its color and its unalterability in the air. Alloyed with silver, it has been employed by dentists as a substitute for gold.

Wiktionary

1. n. A safeguard (from a statue of Athena that was believed to safeguard the ancient city of Troy).
2. n. A metallic chemical element (symbol Pd) with an atomic number of 46.

GNU Webster's 1913

1. n. (Gr. Antiq.) Any statue of the goddess Pallas; esp., the famous statue on the preservation of which depended the safety of ancient Troy.
2. n. That which affords effectual protection or security; a safeguard.
3. n. (Chem.) A rare metallic element of the light platinum group, found native, and also alloyed with platinum and gold. It is a silver-white metal resembling platinum, and like it permanent and untarnished in the air, but is more easily fusible, with a melting point of 1555° C. It can also be prepared as a finely divided black powder. It is unique in its power of absorbing hydrogen, which it does to the extent of nearly a thousand volumes, forming the alloy Pd2H. It is used for graduated circles and verniers, for plating certain silver goods, and somewhat in dentistry. It was so named in 1804 by Wollaston from the asteroid Pallas, which was discovered in 1802. Symbol Pd. Atomic number, 46. Atomic weight, 106.42. Density 12.0.

WordNet 3.0

1. n. a silver-white metallic element of the platinum group that resembles platinum; occurs in some copper and nickel ores; does not tarnish at ordinary temperatures and is used (alloyed with gold) in jewelry