Bitcoin (abbreviation: BTC[a] or XBT;[b] sign: ₿) is a decentralized digital currency. Bitcoin transactions are verified by network nodes through cryptography and recorded in a public distributed ledger called a blockchain. The cryptocurrency was invented in 2008 by an unknown entity under the name Satoshi Nakamoto. The currency began use in 2009, when its implementation was released as open-source software.: ch. 1 The word "bitcoin" was defined in a white paper published on October 31, 2008.It is a compound of the words bit and coin. The Library of Congress reports that, as of November 2021, nine countries have fully banned bitcoin use, and a further forty-two have implicitly banned it.In contrast, a few governments have used bitcoin in some capacity. For example, El Salvador has adopted Bitcoin as legal tender, although use by merchants remains low.Ukraine has accepted cryptocurrency donations to fund the resistance to the 2022 Russian invasion, and Iran has used bitcoin to bypass political sanctions. Bitcoin has been described as an economic bubble by at least eight recipients of the Nobel Memorial Prize in Economic Sciences. As of 2022, bitcoin mining was estimated to be responsible for 0.1% of world annual greenhouse gas emissions. According to the University of Cambridge, bitcoin has emitted an estimated 200 million tonnes of carbon dioxide since its launch,or about 0.04% of all carbon dioxide released since 2009.