Confirmation

A confirmation is a block that includes your transaction (or is mined on top of the block that does). The moment a transaction lands in a block it has one confirmation, said to be "mined at a depth of 1 block." Every subsequent block found by the network adds another confirmation, burying the transaction deeper under accumulated proof-of-work and making any attempt to reverse it progressively more expensive.

Why depth equals security

An unconfirmed (0-conf) transaction sitting in the mempool can still be displaced by a conflicting one through a race or Finney attack. Once it is buried under blocks, an attacker would have to re-mine every block from the transaction forward and out-pace the honest network, an effort whose cost climbs sharply with each added block.

The six-confirmation convention

The classic Bitcoin client treats six confirmations as practically settled, a threshold chosen on the assumption an attacker controls well under 10% of hashrate. Six blocks take roughly an hour. For high-value settlement, exchanges often wait for more, and 144 blocks (about one day) is a common conservative benchmark. The number is a risk choice, not a protocol rule, because Bitcoin offers no absolute moment of settlement.

Confirmations are the practical face of Bitcoin's probabilistic finality. The deeper a transaction sits, the closer it gets to irreversible, though it is the work of difficulty adjustment that keeps blocks arriving near the ten-minute target so confirmation times stay predictable.