An altcoin is any cryptocurrency other than Bitcoin - the term is short for "alternative coin".
Altcoins include both large, established projects such as Ethereum, Solana or Cardano, as well as thousands of smaller coins that are often highly speculative and have low liquidity.
Some altcoins pursue a genuinely distinct technical approach (such as smart-contract platforms or privacy coins), while others are simple forks or copies of existing projects with no meaningful innovation.
Because altcoins are generally much more volatile and less liquid than Bitcoin, they are considered riskier overall – particularly smaller, little-known altcoins with low trading volume ("microcaps").
