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 legal tender token for commerce in the designated country, region, or territory.

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 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 most money systems, the highest value coin made for circulation is worth less than the lowest-value note. The face value of circulation coins is usually higher than the gross value of the metal used in making them, but this is not generally the case with historical 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 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 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).