A wooden block fixed on back side of a door frame on its post, is known as

Definition: A cleat is a small wooden block fixed to the back side of a door frame (on its post) to aid in fixing the frame to the surrounding wall or structural support.

Purpose: It provides a firm fixing point during installation, keeping the door frame stable and correctly aligned until it is permanently anchored.

Not a stop: A stop is a strip along the inside of the frame that the door shuts against; it prevents the door from swinging through, not for fixing the frame.

Not a horn: A horn is a projection/extension of the frame beyond the jamb used for bonding into masonry or for appearance; it is not a separate fixing block.