Spare sector
Extra sectors a disk holds in reserve to replace ones found defective, keeping the address space looking whole from outside.
No magnetic surface is perfect. Weak sectors are mapped at manufacture into a defect list (the P-list), and more surface during use and land in the G-list. Rather than leave holes, the disk reserves extra sectors per track or per zone and quietly remaps a bad address to a healthy spare — sector sparing.
On older disks this happened at low-level formatting; modern hard drives and SSDs do it invisibly in firmware, which is why a disk can "heal" a few bad blocks before SMART counters start climbing. Once the spares run out, the errors begin to show.