What is Reorganisation (reorg)?
When nodes switch to a different chain with more accumulated work, discarding one or more recently accepted blocks.
Short reorgs of one block happen naturally when two miners find blocks at nearly the same moment; the network converges on whichever chain gets extended first.
Deep reorgs would require enormous sustained hashrate and have not occurred on Bitcoin outside its earliest days. This is the mechanism confirmations protect against.
Related terms
Confirmation
A block containing your transaction. Each subsequent block adds another, making reversal exponentially more…
How it worksProof of work
The mechanism that secures Bitcoin: miners must find a hash below a difficulty target, which is expensive to…
Network51% attack
An attack in which someone controlling a majority of hashrate reorganises recent blocks to reverse their own…
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.