A coin is a piece of hard material that is standardized in weight, is produced in large quantities in order to facilitate trade, and primarily can be used as a medium of exchange or legal tender.
Coins are usually metal or a metallic material and sometimes made of synthetic materials, usually in the shape of a disc, and most often issued by a government. Coins are used as a form of money in transactions of various kinds, from the everyday circulation coins to the storage of large numbers of bullion coins. In the present day, coins and banknotes make up currency, the cash forms of all modern money systems. Coins made for paying bills and general monetized use are usually used for lower-valued units, and banknotes for the higher values; also, in many money systems, the highest value coin made for circulation is worth less than the lowest-value note. In the last hundred years, the face value of circulation coins has usually been higher than the gross value of the metal used in making them; exceptions occurring when inflation causes the metal value to surpass the face value, causing the minting authority to change the composition and the old coins to begin to disappear from circulation (see Gresham's Law.) However, this has generally not been the case throughout the rest of history for circulation coins made of precious metals.
Exceptions to the rule of coin face-value being higher than content value, also occur for some bullion coins made of silver or gold (and, rarely, other metals, such as platinum or palladium), intended for collectors or investors in precious metals. Examples of modern gold collector/investor coins include the American Gold Eagle minted by the United States, the Canadian Gold Maple Leaf minted by Canada, and the Krugerrand, minted by South Africa. The American Gold Eagle has a face value of US$50, and the Canadian Gold Maple Leaf coins also have nominal (purely symbolic) face values (e.g., C$50 for 1 oz.); but the Krugerrand does not.
Historically, a great number of coinage metals (including alloys) and other materials (e.g. Porcelain) have been used practically, artistically, and experimentally in the production of coins for circulation, collection, and metal investment, where bullion coins often serve as more convenient stores of assured metal quantity and purity than other bullion.
Coins have long been linked to the concept of money, as reflected by the fact that in some other languages the words "coin" and "currency" are synonymous. Fictional currencies may also bear the name coin (as such, an item may be said to be worth 123 coin or 123 coins).