Replace-by-Fee (RBF)

Replace-by-Fee is a Bitcoin feature that lets the original sender of an unconfirmed transaction rebroadcast a new version of that same transaction with a higher fee, which the network treats as replacing the original — the new, higher-fee transaction becomes the one eligible for confirmation, while the original lower-fee version is dropped. This gives a sender a way to speed up a transaction stuck in the mempool (see Kaavon's "Mempool" glossary definition) due to an initially too-low fee, without needing to wait indefinitely for the original to confirm.

A transaction must be explicitly flagged as RBF-eligible when first broadcast for the sender to later replace it this way, since not every wallet or transaction supports it by default. RBF has sometimes been criticized as making zero-confirmation transactions — payments accepted before any block confirmation — less reliable for a recipient, since a sender could in principle replace a transaction with a different one sending funds elsewhere before the original ever confirms, though this is a long-recognized limitation of accepting unconfirmed transactions generally, not unique to RBF.