A coin is a piece of hard material used primarily as a medium of exchange or legal tender. They are standardized in weight, and produced in large quantities at a mint in order to facilitate trade. They are most often issued by a government.
Coins are usually metal or alloy, or sometimes made of synthetic materials. They are usually disc shaped. Coins made of valuable metal are stored in large quantities as bullion coins. Other coins are used as money in everyday transactions, circulating alongside banknotes. Usually the highest value coin in circulation is worth less than the lowest-value note. In the last hundred years, the face value of circulation coins has occasionally been lower than the value of the metal they contain, for example due to inflation. If the difference becomes significant, the issuing authority may decide to withdraw these coins from circulation, or the public may decide to melt the coins down or hoard them . (source : wikipedia