Definition of 'Bitcoin'
Bitcoin is a digital currency created in 2009. It follows the ideas set out in a white paper by the mysterious Satoshi Nakamoto, whose true identity has yet to be verified. Bitcoin offers the promise of lower transaction fees than traditional online payment mechanisms and is operated by a decentralized authority, unlike government issued currencies.
There are no physical Bitcoins, only balances associated with public and private keys. These balances are kept on a public ledger, along with all Bitcoin transactions, that is verified by a massive amount of computing power.
Investopedia explains 'Bitcoin'
Bitcoin balances are kept using public and private "keys," which are long strings of numbers and letters linked through the mathematical encryption algorithm that was used to create them. The public key (comparable to a bank account number) serves as the address which is published to the world and to which others may send Bitcoin. The private key (comparable to an ATM PIN) is meant to be a guarded secret, and only used to authorize Bitcoin transmissions.
In March 2014, the IRS stated that all virtual currencies, including Bitcoin, would be taxed as property rather than currency. Gains or losses from Bitcoin held as capital will be realized as capital gains or losses, while Bitcoin held as inventory will incur ordinary gains or losses.
The independent individuals and companies who own the governing computing power and participate in the network, also known as "miners," are motivated by mining rewards (the release of new Bitcoin) and transaction fees paid in Bitcoin. These miners can be thought of as the decentralized authority enforcing the credibility of the Bitcoin network. New Bitcoin is being released to the miners at a fixed, but periodically declining rate, such that the total supply of Bitcoin approaches 21 million. One bitcoin is divisible to eight decimal places (100 millionth of one bitcoin), and this smallest unit is referred to as a Satoshi. If necessary, and if the participating miners accept the change, Bitcoin could eventually be made divisible to even more decimal places.
Style notes: According to the official Bitcoin Foundation, the word "Bitcoin" is capitalized in the context of referring to the entity or concept, whereas "bitcoin" is written in the lower case when referring to a quantity of the currency (e.g. "I traded 20 bitcoin"). The currency can be abbreviated to BTC or, less frequently, XBT. The plural form of the word can be either "bitcoin" or "bitcoins."
To learn more about this cryptocurrency, check out How do I buy Bitcoins?