Glossary
Replace-By-Fee (RBF)
A Bitcoin policy (BIP-125) that lets a sender replace an unconfirmed transaction with a higher-fee version to speed up confirmation.
When a Bitcoin transaction is stuck in the mempool with a fee too low for the current block space market, RBF lets the sender broadcast a new transaction that spends the same inputs but pays a higher fee. Miners replace the original with the bumped version.
The original transaction must signal RBF (by setting at least one
input's sequence number below 0xfffffffe). Most modern wallets
opt in by default; some merchants who accept zero-confirmation
payments refuse RBF-signaling transactions for their (very weak)
double-spend protection.