Altcoin is an abbreviation of "Bitcoin Alternative." In this regard, the term altcoin describes every single cryptocurrency which is not Bitcoin.
There are thousands of altcoins 1100+ listed in coinmarketcap (and much more in existence but not listed on the major exchanges), and their number is growing exponentially every day.
Most altcoins are simple Bitcoin clones, changing only minor parameters in the code such as its hashing algorithm, transactions speed, distribution method. Most of these altcoins that are not offering any significant innovation do not survive for very long.
But there are some "altcoins" which although employing the blockchain technology are very different from Bitcoin in many aspects.
The first altcoin ever created was Namecoin, created in April 2011. Its primary function - apart from serving as a currency - is to decentralize domain-name registrations, to make internet censorship more difficult.
Altcoins can be obtained in all major exchanges and are playing a vital role in the cryptocurrency ecosystem.
Some notable altcoins are Ethereum, Ripple, Litecoin, Dash, Nem, Monero, Zcash, Bitcoin Cash and others.
Image: Flickr by Namecoin Cryptocurrency Art Gallery