An altcoin is any cryptocurrency other than Bitcoin (sometimes other than BTC and ETH).
Altcoins range from smart-contract L1s to memes. Risk and liquidity vary enormously.
In research workflows, Altcoin is usually interpreted with Mid-cap altcoin, Bitcoin, Ethereum, and Altcoin season rather than as a standalone signal. Narrative, liquidity, and rank dynamics matter as much as branding. Verify contracts before you transfer.