confirmation

When a Bitcoin transaction is broadcast to the network, it enters the mempool and waits to be included in a block. The moment a miner includes it in a mined block and that block is added to the blockchain, the transaction receives its first confirmation. Each subsequent block added on top of that block adds another confirmation. Confirmations are a measure of how deeply a transaction is buried in the blockchain, and therefore how much computational work would be required to reverse it.

The security of a transaction increases with each confirmation because reversing it would require rewriting not just the block it is in, but all the blocks added since. This is what makes a 51% attack, where a single actor controls the majority of the network's mining power, progressively more costly and impractical as confirmations accumulate. The probability that a transaction will be reversed drops sharply after even a few confirmations. For small transactions, one confirmation is often considered sufficient. For large transfers, many recipients and exchanges wait for six confirmations, a threshold that has become an industry standard, though not a protocol rule.

Block times in Bitcoin average approximately ten minutes, so six confirmations take roughly one hour. This is not a hard finality guarantee but a practical security threshold. For very large transfers, some counterparties wait for more confirmations. The Lightning Network, Bitcoin's second-layer payment protocol, provides faster settlement by routing payments off-chain, avoiding the need to wait for on-chain confirmations for everyday transactions.