Definition
Confirmations depth is the count of blocks built on top of the block that contains a given transaction, including that block itself. A transaction in the most recent block has one confirmation; once five more blocks are stacked above it, it has six. Depth is the practical measure of how settled a transaction is, because reversing it would require re-mining its block and every block above it.
Why depth equals security
Each additional block represents real proof-of-work that an attacker would have to redo and then outpace to rewrite history. Since hashpower is finite and the honest chain keeps extending, the probability that an attacker can produce a longer competing chain falls exponentially as depth increases. This is why a transaction with many confirmations is treated as effectively irreversible even though, in principle, no Bitcoin transaction is ever absolutely final.
Choosing a required depth
Exchanges and merchants pick a confirmation threshold proportional to the value and counterparty risk. Small, low-risk payments may be accepted at one confirmation or even zero; large transfers commonly wait for six; some institutions require more during periods of unusual hashrate concentration. The right number is a risk-management decision, not a fixed protocol constant.
For the theory and edge cases, see probabilistic finality and reorg protection.
In Simple Terms
Confirmations depth is the count of blocks built on top of the block that contains a given transaction, including that block itself. A transaction in…