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A coin is a piece of hard material, usually metal and usually in the shape of a disc, used as a form of money.

Coins were first used in Lydia in prehistoric times. Until relatively recent times, a coin was simply a piece of valuable metal (often gold or silver) whose weight was guaranteed by a government or other authority, whose value depended on its having a known weight of valuable material. The milled edges still found on many coins were designed to show that none of the valuable metal had been shaved off the coin. Now, however, coins derive value simply by virtue of being declared legal tender by a government.

The practice of debasing the currency by alloying the gold or silver with less valuable metals, in order to mint coins that had a lower intrinsic value, goes back at least to the Roman empire.

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