What is Bullion?

Legal Definition
Bullion is gold bars, silver bars, and other bars or ingots of precious metal. The word bullion comes from the French Minister of Finance under Louis XIII, Claude de Bullion.

The value of bullion is typically determined by the value of its precious metals content, which is defined by its purity and mass. To assess the purity of gold bullion, the centuries old technique of fire assay is still employed, together with modern spectroscopic instrumentation, to accurately determine its quality in order and ensure the owner receives fair market value for it. It is also weighed extremely accurately.

Retailers may sometimes market ingots and bars of base metals, such as copper, nickel, and aluminium as bullion, but this is not a widely accepted definition.
-- Wikipedia
Legal Definition
In its usual acceptation, is uncoined gold or silver, in bars, plates, or other masses. 1 East, P. C. 188.

2. In the acts of Congress, the term is also applied to copper properly manufactured for the purpose of being coined into money. For the acts of Congress, authorizing the coinage of bullion for private individuals, see Act of April 2, 1792, s. 14, 1 Story, 230; Act of May 19, 1828, 4 Sharsw. cont. of Story's Laws U. S. 2120; Act of June 28, 1834, Id. 2376; Act of January 18, 1837, Id. 2522 to 2529. See, for the English law on the subject of crimes against bullion, 1 Hawk. P. C. 32 to 41.
-- Bouviers Law Dictionary
Legal Definition
Gold and silver intended to be coined. The term is usually applied to a quantity of these metals ready for the mint, but as yet lying in bars, plates, lumps or other masses; but it may also include ornaments or dishes of gold and silver or foreign coins not current as money, when intended to be descriptive of its adaptability to be coined, and not of other purposes to which it may be put. Hope Min. Co. v. Kennon, 3 Mont. 44; Thalheim v. State, 38 Fla. 169, 20 South. 938; Counsel v. Min. Co., 5 Daly (N. Y.) 77. See Bullion fund.
-- Black's Law Dictionary
Legal Definition
Uncoined gold and silver.
-- Ballentine's Law Dictionary