Also called: time-delayed standby, lagging replica
In plain English
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.
Why it matters
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.