Quick Answer: A coin is a small object, usually round and flat, 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 often have images, numerals, or text on them. The faces of coins or medals are sometimes called the obverse and the reverse, referring to the front and back sides, respectively. The obverse of a coin is commonly called heads, because it often depicts the head of a prominent person, and the reverse is known as tails. The first metal coins – invented in the ancient Greek world and disseminated during the Hellenistic period – were precious metal–based, and were invented in order to simplify and regularize the task of measuring and weighing bullion carried around for the purpose of transactions.