Bitcoin is a peer-to-peer payment system introduced as open source software in 2009 by developer Satoshi Nakamoto.[4] The payments in the system are recorded in a public ledger using its own unit of account,[5] which is also called bitcoin.[note 1] While not considered a true currency, bitcoin is commonly referred to as a digital currency, virtual currency, electronic money, or cryptocurrency. The bitcoin system is not controlled by a single entity, like a central bank, which has led the US Treasury to call bitcoin a decentralized currency.[8] Because bitcoins can be transferred directly from one person to another they are sometimes described as digital cash.[9]