A “reorg” in the context of blockchain technology refers to a “blockchain reorganization”. This is an event where one chain becomes longer than the one currently accepted by the network, causing the nodes to switch to the longer chain. The term reorg is shorthand for reorganization.
This is a natural part of how blockchain works. For example, Bitcoin’s blockchain allows for temporary forks due to the way it handles network latency. It’s possible for two miners to solve a block at roughly the same time, leading to two competing chains. The network then decides which chain to continue with based on the “longest chain rule”, where the chain with the most proof-of-work (i.e., the longest chain) is deemed the correct one. Once the decision is made, the blockchain “reorganizes” to this new longest chain.
However, when reorgs happen deep in the blockchain’s history, they could potentially result in a double-spending attack. In such a situation, a nefarious actor could trick the network into accepting a transaction, wait for services or goods to be delivered, then create a longer chain where that transaction never occurred. This is generally prevented in most blockchains by a combination of measures, including the computational difficulty of rewriting multiple blocks and the economic incentives of following the protocol honestly.
It’s also worth noting that not all blockchains handle reorganizations in the same way. For example, in Ethereum, reorgs (known as “uncles”) contribute to the security of the network and miners of these blocks receive partial rewards.