Altcoin is simply the term used for Cryptocurrency other than Bitcoin.
For instance, cryptocurrencies like Matic, Cardano, XRP, Caspa, and Hedera came into existence following the widespread popularity and success of Bitcoin.
Namecoin is the first altcoin launched in 2011, and, by now, there are 5000+ of them.
Early altcoins focused on enhancing specific aspects of Bitcoin, such as transaction speed and energy efficiency. In contrast, more recent altcoins serve a variety of purposes depending on the goals of the developers.