Bitcoin^{[note 5]} is an online payment system invented by Satoshi Nakamoto,^{[note 6]} who published his invention in 2008,^[12] and released it as open-source software in 2009.^[13] The system is peer-to-peer; users can transact directly without needing an intermediary.^[14]:4 Transactions are verified by network nodes and recorded in a public distributed ledger called the block chain.^{[10]:ch. 6[15]} The ledger uses its own unit of account, also called bitcoin.^{[note 7]} The system works without a central repository or single administrator, which has led the US Treasury to categorize it as a decentralized virtual currency.^[1] Bitcoin is often called the first cryptocurrency,^[19] although prior proposals existed.^{[note 8]} Bitcoin is more correctly described as the first decentralized digital currency.^[14][22] It is the largest of its kind in terms of total market value.^[23]

Bitcoins are created as a reward for payment processing work in which users offer their computing power to verify and record payments into the public ledger. This activity is called mining and is rewarded by transaction fees and newly created bitcoins.^[14] Besides mining, bitcoins can be obtained in exchange for different currencies,^[24] products, and services.^[25] Users can send and receive bitcoins for an optional transaction fee.^[26]

Bitcoin as a form of payment for products and services has grown,^[25] and merchants have an incentive to accept it because fees are lower than the 2–3% typically imposed by credit card processors.^[27] Despite a big increase in the number of merchants accepting bitcoin, the cryptocurrency doesn’t have much momentum in retail transactions.^[28] Unlike credit cards, any fees are paid by the purchaser, not the vendor. The European Banking Authority^[29] and other sources^[14]:11 have warned that bitcoin users are not protected by refund rights or chargebacks.

The use of bitcoin by criminals has attracted the attention of financial regulators,^[30] legislative bodies,^[31] law enforcement,^[32]and media.^[33] They listed money laundering, financing of illicit activities, theft, fraud, tax evasion, and use in black markets as possible. As of 2013, the criminal activities centered around theft and black markets. Officials in countries such as the United States also recognized that bitcoin can provide legitimate financial services.^[34]