Bitcoin is a decentralized cryptocurrency originally introduced in a white paper in 2008 by a person, or group of people, using the alias Satoshi Nakamoto. It was launched shortly after, in January 2009.
Bitcoin is an online peer-to-peer currency, which means that all transactions occur directly between participants, equal and independent, of the network without the need for any intermediary to grant permission or facilitate the operations. Bitcoin was created, according to Nakamoto's own words, to allow "online payments to be sent directly from one party to another without going through a financial institution."
There are some concepts that describe a similar type of decentralized electronic currency before BTC, but Bitcoin has the distinction of being the first cryptocurrency to come into use.
