A stable coin is a cryptocurrency that is pegged to another stable asset, like gold or the U.S. dollar. Its a currency that is global, but is not tied to a central bank and has low volatility. This allows for practical usage of using cryptocurrency.
On the other hand, an optimal cryptocurrency should have the following four traits: price stability, scalability, privacy, and decentralization. However, stability is key. Short-term stability is important for transactions and long-term stability is important for holding.