The generally agreed-upon definition of an altcoin is any other cryptocurrency that is not Bitcoin - simple as that. In other words, all of the different cryptocurrencies (except for Bitcoin) that you can find on the market are going to be altcoins! Sure, the definition tends to change a bit with time, but the general idea remains the same - say, if you were to buy BNB on KuCoin or any other exchange, you would actually be buying.