Altcoin is a term used to refer to cryptocurrencies other than Bitcoin. Altcoin stands for "alternative coin" and includes thousands of different cryptocurrencies, each with its own unique features and technologies. Some well-known altcoins include Ethereum, Ripple (XRP), Litecoin, and Cardano. These altcoins are often developed to improve or add functionalities to Bitcoin, such as faster transaction speeds, higher security, or the ability to support smart contracts and decentralized applications (DApps).