A coin whose worth is determined by the value of its component metal rather than any symbolic ‘minted’ value. A bullion coin is a way of holding gold by buying physical amounts of it from a mint.
However, it is also possible to hold spendable gold online or via your smartphone. This negates costs and concerns over storage and security and allows you to spend your gold instantly anywhere in the world, giving you complete liquidity, something not given by bullion coins because their given value does not correspond to the value of the metal that makes them up.
A bullion coin on display at the Trial of the Pyx