What is Confirmation?
A block containing your transaction. Each subsequent block adds another, making reversal exponentially more expensive.
One confirmation means included in the most recent block. Six is a common convention for large amounts, but it is a convention rather than a protocol rule.
The right number depends on what is at stake. Reversing a transaction requires out-mining the network from before it, so the appropriate depth scales with the value and the plausibility of an attacker.
Related terms
Block
A batch of transactions accepted into the ledger together, produced roughly every ten minutes and limited to…
How it worksReorganisation (reorg)
When nodes switch to a different chain with more accumulated work, discarding one or more recently accepted…
How it worksTransaction
A signed instruction consuming one or more UTXOs and creating new ones. Once confirmed it cannot be reversed.
How it worksDouble-spend
Spending the same coins twice. Preventing it without a trusted third party is the problem Bitcoin was built…
This definition is part of the Bitcoin Logical glossary. For the fuller explanation, start with Learn, or see the live numbers on The Bitcoin Signal.