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. Transactions are verified by network nodes through cryptography and recorded in a public distributed ledger called a blockchain.
Bitcoin is unique in that there are a finite number of them: 21 million.
Bitcoins are created as a reward for a process known as mining. They can be exchanged for other currencies, products, and services.
 As of February 2015, over 100,000 merchants and vendors accepted bitcoin as payment..
Research produced by University of Cambridge estimates that in 2017, there were 2.9 to 5.
8 million unique users using a cryptocurrency wallet, most of them using bitcoin..
The first wallet program was released in 2009 by Satoshi Nakamoto as open-source code. Unlike traditional fiat currencies, which are issued by central banks, Bitcoin has no central monetary authority. Instead it relies on an peer-to-peer network made up of its users’ computers to verify and record transactions. In the case of Bitcoin, users send payments by broadcasting digitally signed messages to the network using bitcoin cryptocurrency wallet software.
Transactions are recorded into a distributed, replicated public database known as the blockchain, with consensus achieved by a proof-of-work system called “mining”. Satoshi Nakamoto wrote the Bitcoin code in C++ and released it in January 2009 on SourceForge under an open source MIT License..
In January 2009, Nakamoto released the first bitcoin software that launched the network and the first units of the bitcoin cryptocurrency called bitcoins. Mining is a record-keeping service done through the use of computer processing power.[f] Miners keep the blockchain consistent, complete, and unalterable by repeatedly grouping newly broadcast transactions into a block, which is then broadcast to the network and verified by recipient nodes. Each block contains a cryptographic hash of the previous block, using the SHA-256 hashing algorithm,:ch.
7 which links it to the previous block,:ch. 7 so that if one block is tampered with it would be very difficult to change all subsequent blocks.