Delayed standby
Also called: time-delayed standby, lagging replica
A delayed standby is a replica deliberately held a fixed interval behind the primary. It keeps receiving and writing WAL in real time, but postpones applying it by the configured delay, so its visible data is, say, an hour in the past while all the recent WAL sits safely on its disk.
What this means
A replica kept a fixed amount of time behind the primary on purpose. It still receives and stores all the WAL in real time, it just holds off applying it by the set delay. So its visible data might be an hour in the past, while the recent changes sit safely on its disk, ready if you need to recover to just before a mistake.
Why it matters operationally
It's a built-in undo button for human error. If someone drops a table or runs a bad UPDATE on the primary, you have the whole delay window to pause replay and recover the pre-disaster state from the delayed standby before the damage is applied there.