When a valid transaction from one chain is maliciously or accidentally broadcast and accepted on another chain after a fork.
A replay attack occurs after a chain split when a transaction valid on one chain can also be validated on the other. If signatures and scripts are compatible, an attacker can rebroadcast the same signed transaction to spend funds on both chains without the sender’s consent.
"After a contentious hard fork, Alice sends coins on Chain A. Without replay protection, the same signed transaction could be replayed on Chain B, unintentionally moving her coins there as well."
Mitigations include protocol-level replay protection, using different address formats, adding chain-specific bits to signatures or scripts, and wallet best practices like splitting coins before spending.
"Wallets may provide a coin-splitting tool to ensure outputs become valid on only one chain, preventing replays when spending."
All terms and definitions may update as the Cryptionary improves.