An altcoin is a cryptocurrency that competes with Bitcoin, which was once the only cryptocurrency available. During its early years, Bitcoin dominated the market to the point where other cryptocurrencies were compared to it. It was all about Bitcoin – and nothing else. So anything that wasn't Bitcoin was sarcastically dubbed altcoins or, even less charitably, in reference to the original digital money.
Any cryptocurrency that isn't Bitcoin is referred to as an altcoin. This kind of categorization made a lot of sense in the early days of cryptocurrency, when Bitcoin commanded the majority of the market's attention and competitors numbered in the dozens, if not hundreds. The number of individual cryptocurrencies is now estimated to reach over 10,000, according to experts. As a result, defining the crypto industry as "Bitcoin and everything else" makes less sense than ever.