Bitcoin is a software-based online payment system described by Satoshi Nakamoto in 2008 and introduced as open-source software in 2009. Payments are recorded in a public ledger using its own unit of account, which is also called bitcoin. Payments work peer-to-peer without a central repository or single administrator, which has led the US Treasury to call bitcoin a decentralized virtual currency. Media reports often refer to bitcoin as a cryptocurrency or digital currency. The status of bitcoin as a currency is disputed. Bitcoin is the first fully implemented decentralized cryptocurrency; most other cryptocurrencies are similar and derived from it. It is also the largest cryptocurrency in terms of total market value. Bitcoins are created as a reward for processing work in which users offer their computing power to verify and record payments into the public ledger. This activity is called mining, which individuals or companies engage in, in exchange for transaction fees and newly created bitcoins. Besides mining, bitcoins can be obtained in exchange for fiat money, products, and services.