Simply put, a coin is a round shaped object that is also metallic. It is created with the purpose of having and transferring currency, says About.com coins expert Susan Headley.
According to Headley, special edition currencies are made for collectors, and these are referred to as "proofs," but not all “coins” can legitimately be labeled as such. Other objects are made that look like coins, but they are called "rounds." The reason for this term is because these rounds look very similar to coins; nonetheless, they are not made to be used as currency, so they cannot legitimately be called a coin.