Well, altcoins are simply coins that are not bitcoins. The word altcoins have been coined here in the bitcointalk forum because this is a bitcoin forum. It stands for alternative coins. the same case can also be said if there is a coin that you like to be your main coin that would make bitcoin an altcoin also like others. For example, you like NEO, you can make it as your main coin and call bitcoin an altcoin. But of course it will be a different case if you include other people's thoughts since we all have different opinions and point of views. Meaning, some would agree with your thought and some won't. Hope this helps.