Definitions
An altcoin is any cryptocurrency that is not Bitcoin. As Bitcoin was the original cryptocurrency, any cryptocurrency that was created afterwards is treated as an “alternative”.
The term altcoin originated when there was a much larger gap between bitcoin and the other cryptocurrencies in terms of adoption, market cap and trust.
At the time, cryptocurrencies other than bitcoin were referred to as altcoins (or alternative coins).
The term has stuck among cryptocurrency traders and is still used to refer to lower cap cryptocurrencies.