An altcoin is any cryptocurrency that is not Bitcoin. The word comes from "alternative coin" because early crypto markets treated Bitcoin as the original and everything else as an alternative.
Examples of altcoins
Altcoins include platform coins such as Ethereum, payment coins such as Litecoin, privacy coins such as Monero, and meme coins such as Dogecoin. Some altcoins are independent coins, while others are tokens issued on another blockchain.
Why altcoins exist
Altcoins often try to improve speed, privacy, programmability, fees, governance, or developer features. They can also be speculative projects with little long-term value, so the label alone does not say whether a coin is useful.
When evaluating an altcoin, look at what problem it solves, who maintains it, how the token is issued, and whether the network has real users.