What is Bullion Coin?

Legal Definition
A bullion coin is a coin struck from precious metal and kept as a store of value or an investment, rather than used in day-to-day commerce. The United Kingdom defines investment coins more specifically as coins that have been minted after 1800, have a purity of not less than 900 thousandths and are, or have been, legal tender in their country of origin. Under United States law, "coins" that fail the last of these requirements are not coins at all, and must be advertised as "rounds" instead. Bullion coins are usually available in both gold and silver, with the exceptions of the Krugerrand and the Swiss Vreneli which are only available in gold. The American Eagle series is available in gold, silver and platinum, and the Canadian Maple Leaf series is available in gold, silver, platinum and palladium.

Bullion coins are typically available in various weights. These are usually multiples or fractions of 1 troy ounce, but some bullion coins are produced in very limited quantities in kilograms or heavier.

Bullion coins sell for a premium over the market price of the metal on the commodities exchanges. Reasons include their comparative small size and the costs associated with manufacture, storage and distribution. The amount of the premium varies depending on the coin's type and weight and the precious metal. The premium also is affected by prevailing demand.

The ISO currency code for gold bullion is XAU. ISO 4217 includes codes not only for currencies, but also for precious metals (gold, silver, palladium and platinum; by definition expressed per one troy ounce, as compared to "1 USD") and certain other entities used in international finance, e.g. special drawing rights.
-- Wikipedia
Legal Definition
A bullion coin is a coin struck from precious metal and kept as a store of value or an investment, rather than used in day-to-day commerce.