"Altcoin" is shorthand for any cryptocurrency other than Bitcoin — literally an "alternative coin". The label covers an enormous range, from large, established networks like Ethereum to thousands of tiny, illiquid tokens that may exist for only a few months.
Lumping them together can mislead, because the risks vary wildly. A major smart-contract platform and an anonymous meme token are both "altcoins", yet they have almost nothing in common in track record, liquidity or the chance of going to zero.
As a rule, the further down the market-cap rankings a coin sits, the thinner its trading and the sharper its swings. Many altcoins that soared in one cycle never recovered their highs. Size and longevity are not guarantees, but they are at least signals worth weighing.
Worked example
Ethereum, Solana and XRP are all altcoins — as is any of the thousands of smaller tokens that are not Bitcoin.
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This definition is general education, not investment advice. Markets — especially crypto — are volatile and you can lose money. Please read our disclaimer and see our methodology.