Bitcoin is the first decentralized cryptocurrency: a digital asset and payment network that operates without a central bank or administrator, in which ownership is recorded on a public proof of work blockchain maintained by thousands of independent nodes. Among cryptocurrencies it is the original design from which the category emerged, and it remains the asset with the longest track record and, for most of its history, the largest market capitalization.
Bitcoin was described in a white paper published in 2008 under the pseudonym Satoshi Nakamoto, and its network started in January 2009 with the genesis block. Its monetary policy is fixed in software: the total supply can never exceed 21 million bitcoin, each divisible into 100 million units called satoshis. The term is commonly written with a capital B for the network and protocol and in lowercase, or as BTC, for the unit of currency.
How the Bitcoin network works
The Bitcoin network works by having miners compete to add blocks of transactions to a shared chain, while full nodes independently verify every block against the consensus rules. A transaction becomes part of the ledger once a miner includes it in a valid block, and it becomes harder to reverse with every block added afterward.
Mining is Nakamoto consensus based on proof of work: miners repeatedly hash a candidate block header with SHA-256 until the result falls below a target value. The first miner to find a valid hash broadcasts the block and receives the block subsidy plus the fees of the included transactions.
| Parameter | Value | Purpose |
|---|---|---|
| Target block interval | About 10 minutes | Balances propagation delay and throughput |
| Difficulty adjustment | Every 2,016 blocks, about two weeks | Keeps the interval stable as hash rate changes |
| Maximum supply | 21 million BTC | Fixed, predictable monetary policy |
| Smallest unit | 1 satoshi, 0.00000001 BTC | Allows very small amounts |
| Block size limit | 4 million weight units | Limits resource demands on nodes |
| Hash function | SHA-256, applied twice | Proof of work and block linking |
Ownership is tracked in the unspent transaction output model. Instead of account balances, the ledger stores outputs that can each be spent once; a wallet’s balance is the sum of the outputs its keys can unlock, and a payment consumes old outputs and creates new ones.
Supply schedule and the halving
New bitcoin enter circulation only as the block subsidy paid to miners, and this subsidy is cut in half every 210,000 blocks, roughly every four years, in an event called the halving. The schedule makes issuance decline geometrically until it stops entirely around the year 2140.
| Era | Approximate start | Block subsidy |
|---|---|---|
| Launch | 2009 | 50 BTC |
| First halving | 2012 | 25 BTC |
| Second halving | 2016 | 12.5 BTC |
| Third halving | 2020 | 6.25 BTC |
| Fourth halving | 2024 | 3.125 BTC |
As the subsidy shrinks, transaction fees must increasingly pay for security. Whether fees alone will sustain enough mining to protect the network in the long run is an open and genuinely debated question among researchers and developers.
Bitcoin compared with other cryptocurrencies
Bitcoin differs from most other cryptocurrencies in its deliberately conservative design. Its scripting language is intentionally limited, protocol changes are rare and backward compatible, and its main role is to serve as a scarce, censorship-resistant monetary asset rather than a general platform for applications.
| Asset | Consensus | Supply policy | Primary role |
|---|---|---|---|
| Bitcoin | Proof of work | Fixed at 21 million | Store of value, settlement |
| Ethereum | Proof of stake | No fixed cap, variable issuance and burn | Smart contract platform |
| Solana | Proof of stake with proof of history | Inflationary with declining rate | High-throughput applications |
| Stablecoins | Depends on host chain | Issued against reserves | Price-stable payments |
| Memecoins | Depends on host chain | Varies widely | Speculation, community |
Other cryptocurrencies are often grouped as altcoins, a term defined by contrast with Bitcoin. Bitcoin can also be represented on other chains as a wrapped token, but that representation depends on a custodian or bridge and does not carry the same security as native bitcoin.
Using and holding bitcoin
Bitcoin is held through private keys that control outputs on the blockchain, usually managed by a wallet. Whoever controls the keys controls the coins, so the choice between self-custody and a custodial service is the most important decision for any holder.
- Self-custody: a software or hardware wallet holds the keys, backed up by a seed phrase.
- Custodial accounts: an exchange or broker holds the keys and owes the user a balance.
- Exchange-traded products: funds such as spot Bitcoin ETFs give price exposure through ordinary brokerage accounts.
- Lightning Network: a layer 2 payment network that settles small, fast payments off-chain.
- Confirmations: recipients of large amounts commonly wait for several confirmations, often six, before treating a payment as final.
The base layer handles only a limited number of transactions per second, so fees rise when demand is high. Upgrades such as SegWit in 2017 and Taproot in 2021 improved efficiency and privacy, while larger payment volumes increasingly move to layer 2 systems.
Risks, criticisms and misconceptions
Bitcoin’s most visible risk is extreme price volatility: its market price has repeatedly risen and fallen by large percentages within months, which limits its use as a unit of account and makes it unsuitable for money needed in the short term.
- Energy consumption: proof of work uses substantial electricity, a frequent environmental criticism.
- Irreversibility: confirmed payments cannot be recalled, so errors and fraud are hard to remedy.
- Key loss: lost private keys make coins permanently unspendable.
- Pseudonymity, not anonymity: all transactions are public, and addresses can often be linked to identities.
- Regulatory change: tax treatment and permitted services vary by country and continue to evolve.
A common misconception is that Bitcoin is controlled by its developers or miners. In practice, changes take effect only if the economic majority of node operators adopts them, which is why the protocol evolves slowly and contested changes have led to separate chains.
The short version
Bitcoin is digital money that no government or bank issues or controls. A worldwide network of computers keeps one shared record of who owns which coins.
Only 21 million bitcoin will ever exist, which is why many people compare it to digital gold. Owning bitcoin means holding a secret key, much like holding the only key to a safe. Lose the key, and the coins are gone.
Where it makes a difference
A freelance software developer living in a country with high inflation and strict currency controls works for clients abroad. International bank transfers to her take several days and lose a noticeable share to conversion fees, so she agrees with two clients to be paid part of her fees in bitcoin.
She receives payments into a hardware wallet she controls, waits for several confirmations on larger amounts, and keeps her seed phrase written down in a secure place. Payments arrive within an hour regardless of banking holidays. Because the price of bitcoin swings sharply, she converts most of each payment into local spending money within days and holds only a small portion long term, accepting that this portion may lose value as easily as it gains.