Definition
Replay protection is a safeguard added during a chain split that ensures a transaction can be confirmed on only one of the resulting blockchains. Without it, the two chains share identical transaction formats and signatures, so a payment you broadcast on one chain can be copied ("replayed") onto the other by anyone watching, potentially moving your coins on a chain you never intended to spend on.
The Replay Problem
Immediately after a split, your private keys control the same coins on both chains, and a validly signed transaction may satisfy the rules of both. An attacker, or even a careless wallet, can rebroadcast your signed transaction on the second chain, draining the matching balance there. This is purely a consequence of the two chains sharing pre-fork history and signature schemes.
How Protection Is Implemented
The cleanest fix is two-way replay protection baked into signature validation. When Bitcoin Cash split off in 2017, it adopted a new signature hashing scheme that sets a fork-specific flag (SIGHASH_FORKID) inside the signed data. Signatures created with that flag are rejected by the original chain, and original-chain signatures lack the flag and are rejected by the new chain. Because each transaction is cryptographically bound to one chain's rules, replay becomes impossible in either direction. Other splits have used distinct flags or address formats to achieve the same isolation.
Replay protection is what makes a deliberate chain split safe to navigate; its presence or absence is a key thing self-custodians check before moving coins after any fork. Because it alters validity, adding it is itself a change to the consensus rules.
In Simple Terms
Replay protection is a safeguard added during a chain split that ensures a transaction can be confirmed on only one of the resulting blockchains. Without…