A coin can refer to a cryptocurrency that can operate independently or to a single unit of such cryptocurrency.
Coins are cryptocurrencies that operate on their own blockchains and are independent of any other coins. A single unit of such cryptocurrency can also be called a coin. The term is used to distinguish autonomous cryptocurrencies from tokens, which operate on top of their parent blockchain platforms, like Ethereum (ETH).
The first-ever coin on the cryptocurrency market was Bitcoin (BTC), launched in January 2009. It uses a decentralized, geographically distributed ledger, called the blockchain, in order to keep track of all transactions that take place on its network and ensure that nobody can create new coins via means other than the computationally-intensive process of mining.
Afterward, many new coins have appeared, some based on their own blockchains designed from scratch, and some — called forks — based on the blockchain of a different, already existing coin. For example, Bitcoin Cash (BCH) and Bitcoin Gold (BTG) are forks of Bitcoin and, at the moment of their creation, had the same blockchains.
However, they are still considered coins, i.e. independent cryptocurrencies because, after the point of the fork, their blockchains continued operating in complete separation from Bitcoin’s.
In contrast, some platforms, such as Ethereum and EOS (EOS), allow people to create tokens — cryptocurrencies whose operation relies entirely on the operation of the parent blockchain and which would stop being usable if the underlying platform ever went down.