Confirmation
Also · Finality · Reorg
A Bitcoin transaction is confirmed once it sits in a valid block. Each later block adds a confirmation and makes reversing it more expensive.
A transaction has 1 confirmation when a valid block includes it, 2 when another block is mined on top, and so on. Until then it sits in the mempool and can be replaced. Each additional block means that undoing the transaction would require redoing that much more proof of work.
Bitcoin’s finality is therefore probabilistic rather than instant. A reorganisation can replace recent blocks with a longer valid chain, but the deeper a transaction is buried, the less plausible that becomes. How many confirmations to wait for depends on the amount at stake. 6 is a common rule of thumb, not a protocol constant.
In French · Confirmation