Server reconciliation
The netcode step where a client re-applies its still-unacknowledged inputs on top of an authoritative server state, correcting prediction error without reintroducing input lag.
It is the counterpart to client-side prediction: the client numbers every input packet and applies it locally at once, so movement feels instant. The server simulates authoritatively and replies with the true state plus the number of the last input it managed to process. The client discards the acknowledged inputs and replays the ones still pending — on top of the server state.
Without the replay, the server's reply (which lags a round trip behind) would yank the player back into the past on every correction. With it, an error shows only when the prediction actually went wrong. Gabriel Gambetta's "Fast-Paced Multiplayer" series (2014) is the canonical write-up. It is a relative of rollback netcode, which replays both players' inputs.