A coin is usually a piece of hard material, usually metal or a metallic material, 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 vast numbers of bullion coins. In the present day, coins and banknotes make up the cash forms of all modern money systems. Coins made for circulation (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. For example, the historical Eagle contained .48375 troy ounce of gold and has a face value of only ten U.S. dollars, but the market value of the coin, due to its metal content, is now many times the face amount.

Exceptions to the rule of coin face-value being higher than content value, also occur for some non-monetized "bullion coins" made of silver or gold (and, rarely, other metals, such as platinum or palladium), intended for collectors or investors in precious metals. For examples of modern gold collector/investor coins, the United States mints the American Gold Eagle, Canada mints the Canadian Gold Maple Leaf, and South Africa mints the Krugerrand. 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, impractically (i.e., rarely), 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).

Collecting coins

The value of a coin

In terms of its value as a collector's item, a coin is generally made more or less valuable by its condition, specific historical significance, rarity, quality/beauty of the design and general popularity with collectors. If a coin is greatly lacking in any of these, it is unlikely to be worth much. Bullion coins are also valued based on these factors, but are largely valued based on the value of the gold or silver in them. Sometimes non-monetized bullion coins such as the Canadian Maple Leaf and the American Gold Eagle are minted with nominal face values less than the value of the metal in them, but as such coins are never intended for circulation, these value numbers are not market nor fiat values, and are never more than symbolic numbers.