Confirmation
Each new block added on top of the one holding your transaction, deepening it and making reversal far harder.
A transaction gets its first confirmation when it lands in a block. Each block mined afterward adds another confirmation on top. The deeper a transaction is buried, the more computing power an attacker would need to rewrite it, so more confirmations means more security.
Conventions vary. Many exchanges wait for six Bitcoin confirmations, roughly an hour, before crediting a deposit. Others accept one for small amounts. Faster chains and proof-of-stake networks reach 'finality' differently, sometimes within seconds. For large transfers, waiting is cheap insurance.
Related terms
A batch of transactions bundled together, time-stamped, and cryptographically linked to the block before it.
TransactionA signed instruction that moves value or data on a blockchain, recorded permanently once miners or validators confirm it.
FinalityThe point at which a confirmed transaction can no longer be reversed or pushed off the chain.
Block HeightThe number of blocks between a given block and the very first one, used to pinpoint its position.