Bitcoin (abbreviation: BTC; sign: ₿) is the first decentralized cryptocurrency. Based on a free-market ideology, bitcoin was invented in 2008 by Satoshi Nakamoto, an unknown entity (person or persons). Use of bitcoin as a currency began in 2009, with the release of its open-source implementation.  In 2021, El Salvador adopted it as legal tender. It is mostly seen as an investment and has been described by some scholars as an economic bubble. As bitcoin is pseudonymous, its use by criminals has attracted the attention of regulators, leading to its ban by several countries as of 2021.