Replay Attack is an attack in which a third party maliciously tries to cause a fraudulent transfer when the blockchain branches due to a hard fork.
The modus operandi of a replay attack is to target the timing of a hard fork and replay (execute on a different ledger) the transactions listed in the block (ledger), thereby transferring assets to a different ledger that the sender did not intend.
Replay attacks are possible when blockchains diverge due to a hard fork, because the same “secret key” is used for both blockchains.
A countermeasure to replay attacks is called “Replay Protection,” which is a device that allows transactions to be distinguished between each blockchain.
Replay Protection is also implemented in Bitcoin Cash (BCH), which was branched off when Bitcoin (BTC) underwent a hard fork.