Bitcoin[a] (₿) is a cryptocurrency invented in 2008 by an unknown person or group of people using the name Satoshi Nakamoto.[13] The currency began use in 2009[14] when its implementation was released as open-source software.[6]
Bitcoin is a decentralized digital currency, without a central bank or single administrator, that can be sent from user to user on the peer-to-peer bitcoin network without the need for intermediaries.[7] Transactions are verified by network nodes through cryptography and recorded in a public distributed ledger called a blockchain. Bitcoins are created as a reward for a process known as mining. They can be exchanged for other currencies, products, and services.[15] Research produced by the University of Cambridge estimated that in 2017, there were 2.9 to 5.8 million unique users using a cryptocurrency wallet, most of them using bitcoin.[16]
The domain name "bitcoin.org" was registered on 18 August 2008.[19] On 31 October 2008, a link to a paper authored by Satoshi Nakamoto titled Bitcoin: A Peer-to-Peer Electronic Cash System[4] was posted to a cryptography mailing list.[20] Nakamoto implemented the bitcoin software as open-source code and released it in January 2009.[21][22][14] Nakamoto's identity remains unknown.[13] On 3 January 2009, the bitcoin network was created when Nakamoto mined the starting block of the chain, known as the genesis block.[23][24] Embedded in the coinbase of this block was the text "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks".[14] This note references a headline published by The Times and has been interpreted as both a timestamp and a comment on the instability caused by fractional-reserve banking.[25]:18 The receiver of the first bitcoin transaction was cypherpunk Hal Finney, who had created the first reusable proof-of-work system (RPoW) in 2004.[26] Finney downloaded the bitcoin software on its release date, and on 12 January 2009 received ten bitcoins from Nakamoto.[27][28] Other early cypherpunk supporters were creators of bitcoin predecessors: Wei Dai, creator of b-money, and Nick Szabo, creator of bit gold.[23] In 2010, the first known commercial transaction using bitcoin occurred when programmer Laszlo Hanyecz bought two Papa John's pizzas for ₿10,000.[29] Blockchain analysts estimate that Nakamoto had mined about one million bitcoins[30] before disappearing in 2010 when he handed the network alert key and control of the code repository over to Gavin Andresen. Andresen later became lead developer at the Bitcoin Foundation.[31][32] Andresen then sought to decentralize control. This left opportunity for controversy to develop over the future development path of bitcoin, in contrast to the perceived authority of Nakamoto's contributions.[33][32]
The unit of account of the bitcoin system is a bitcoin. Ticker symbols used to represent bitcoin are BTC[b] and XBT.[c][95]:2 Its Unicode character is ₿.[1] Small amounts of bitcoin used as alternative units are millibitcoin (mBTC), and satoshi (sat). Named in homage to bitcoin's creator, a satoshi is the smallest amount within bitcoin representing 1⁄100000000 bitcoins, one hundred millionth of a bitcoin.[2] A millibitcoin equals 1⁄1000 bitcoins; one thousandth of a bitcoin or 100,000 satoshis.[96]
The bitcoin blockchain is a public ledger that records bitcoin transactions.[99] It is implemented as a chain of blocks, each block containing a hash of the previous block up to the genesis block[d] of the chain. A network of communicating nodes running bitcoin software maintains the blockchain.[34]:215–219 Transactions of the form payer X sends Y bitcoins to payee Z are broadcast to this network using readily available software applications.