Bitcoin is a decentralized cryptocurrency that was first outlined in a whitepaper in 2008 by a person or group of persons going by the pseudonym Satoshi Nakamoto. Soon later, in January 2009, it was released.
Bitcoin is a peer-to-peer internet money, which means that all transactions take place directly between equal, independent network members, with no need for any middleman to allow or facilitate them. According to Nakamoto, Bitcoin was developed to enable “online payments to be transferred directly from one party to another without passing via a banking institution.”
Some ideas for a similar kind of decentralized electronic money predate BTC, but Bitcoin is the first cryptocurrency to be used in practice.