An altcoin is any cryptocurrency other than Bitcoin. The term 'alt' means alternative. Altcoins include well-known cryptocurrencies like Ethereum and thousands of newer or smaller cryptocurrencies. Each altcoin typically has its own blockchain, features, and purposes.
Bitcoin is the original cryptocurrency, while Ethereum is an altcoin that allows people to build applications on its blockchain. Dogecoin is another altcoin that started as a joke but developed its own community. All three are cryptocurrencies, but only Bitcoin is not considered an altcoin.
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