Bitcoin is a cryptocurrency and a payment system invented by an unidentified programmer,  or group of programmers, under the name of Satoshi Nakamoto. Bitcoin was introduced on 31 October 2008 to a cryptography mailing list,  and released as open-source software in 2009. There have been various claims and speculation concerning the identity of Nakamoto,  none of which are confirmed. 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,  the U.S. Treasury categorizes bitcoin as a decentralized virtual currency. Bitcoin is often called the first cryptocurrency,  although prior systems existed and it is more correctly described as 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.
In February 2015, the number of merchants accepting bitcoin for products and services passed 100,000.  Instead of 2–3% typically imposed by credit card processors, merchants accepting bitcoins often pay fees in the range from 0% to less than 2%.  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.  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.