The definition of an altcoin has evolved since the early days of cryptocurrency when only a handful of crypto assets existed. Back then, anything but bitcoin (BTC) was considered an altcoin.
Today the world of crypto assets is much broader. There is a wide variety of coins and tokens with use cases that go well beyond a medium of exchange, meaning they aren’t necessarily bitcoin competitors.
Still, for lack of a better designation, the basic definition of an altcoin is any crypto asset other than bitcoin.