An Altcoin is a cryptocurrency other than Bitcoin. Altcoins are created in order to improve on Bitcoin, some are just clones that aren't actually different from BTC, and some try to do completely new things.There are many different types of altcoins, and I believe hashing algorithms, block times, and block sizes are the most important aspects of them, but the idea behind each one is what defines it.The project was inspired by Bitcoin, and it used Bitcoin's code."Altcoin" is a combination of two words: "alt" and "coin". The word "alt" is short for alternative and "coin" means currency.
This nomenclature comes from the idea that bitcoin is the original cryptocurrency and that all others are then considered "alternate" or "alternative" coins.