Bitcoin is far from the only cryptocurrency out there. Today there are thousands of cryptocurrencies. Often they are labelled as altcoins as in alternative coins. And Bitcoin is the original.
Today some see Ethereum, Litecoin and others to belong to the same category as Bitcoin, and no longer an ‘altcoin’. But this is highly subjective.
Often an altcoin can be referred to a new cryptocurrency or coin that has yet to fully establish itself. But as most cryptocurrencies are less than ten years old the entire crypto market could be seen this way.