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Bitcoin is a cryptocurrency and an electronic payment system invented by an unidentified programmer, or group of programmers, under the name of Satoshi Nakamoto. Nakamoto introduced bitcoin on 31 October 2008 to a cryptography mailing list, and released as open-source software in 2009. The system is peer-to-peer, and transactions take place between users directly, without an intermediary. These transactions are verified by network nodes and recorded in a public distributed ledger called the blockchain, which uses bitcoin as its unit of account. Since the system works without a central repository or single administrator, bitcoin is called the first decentralized digital currency. Bitcoin is the largest of its kind in terms of total market value.
Bitcoins are created as a reward in a competition in which users offer their computing power to verify and record bitcoin transactions into the blockchain. This activity is referred to as mining and successful miners are rewarded with transaction fees and newly created bitcoins. Besides being obtained by mining, bitcoins can be exchanged for other currencies, products, and services. When sending bitcoins, users can pay an optional transaction fee to the miners. This may expedite the transaction being confirmed.
As of February 2015, over 100,000 merchants and vendors accept bitcoin as payment. Instead of a 2–3% fee typically imposed by credit card processors, merchants accepting bitcoins often pay fees of 0% to less than 2% of the total purchase. Despite the fourfold increase in the number of merchants accepting bitcoin in 2014, the cryptocurrency did not have much momentum in retail transactions. The European Banking Authority and other sources have warned that bitcoin users are not protected by refund rights or chargebacks.
According to a research produced by Cambridge University in 2017, there are between 2.9 million and 5.8 million unique users actively using a cryptocurrency wallet, most of them using bitcoin. The number of active users has grown significantly since 2013 (there were 0.3 to 1.3 million unique users at the time).
The use of bitcoin by criminals has attracted the attention of financial regulators, legislative bodies, law enforcement, and media. Criminal activities are primarily focused on darknet markets and theft, though officials in countries such as the United States also recognize that bitcoin can provide legitimate financial services.
The unit of account of the bitcoin system is bitcoin. As of 2014, symbols used to represent bitcoin are BTC, XBT, and BitcoinSign.svg. Small amounts of bitcoin used as alternative units are millibitcoin (mBTC), microbitcoin (µBTC, sometimes referred to as bit), and satoshi. Named in homage to bitcoin's creator, a satoshi is the smallest amount within bitcoin representing 0.00000001 bitcoin, one hundred millionth of a bitcoin. A millibitcoin equals to 0.001 bitcoin, one thousandth of a bitcoin. One microbitcoin equals to 0.000001 bitcoin, one millionth of a bitcoin.
A proposal was submitted to the Unicode Consortium in October 2015 to add a codepoint for the symbol. As of November 2016, it is in the pipeline for position 20BF (₿) in the Currency Symbols block.
The United States dollar (sign: $; code: USD; also abbreviated US$ and referred to as the dollar, U.S. dollar, or American dollar) is the official currency of the United States and its insular territories per the United States Constitution. It is divided into 100 smaller cent (¢) units. The circulating paper money consists of Federal Reserve Notes that are denominated in United States dollars (12 U.S.C. § 418).
The U.S. dollar is commodity money of silver as enacted by the Coinage Act of 1792 which determined the dollar to be 371 4/16 grain (24.1 g) pure or 416 grain (27.0 g) standard silver. Since the currency is the most used in international transactions, it is the world's primary reserve currency. Several countries use it as their official currency, and in many others it is the de facto currency. Besides the United States, it is also used as the sole currency in two British Overseas Territories in the Caribbean: the British Virgin Islands and Turks and Caicos Islands. A few countries use the Federal Reserve Notes for paper money, while still minting their own coins, or also accept U.S. dollar coins (such as the Susan B. Anthony dollar).