Spare core (redundancy)
Beginner
An extra copy of a small part of the chip, kept in reserve so it can take over if the original turns out to be broken.
Novice
Extra cores (or memory rows, or links) built into a design. After testing, broken ones are switched off and spares are wired in their place, so a chip with a few defects still works.
Expert
Repair by remapping: test finds failing units, fuses or configuration record the map, and the network routes around them. Spares cost area and power up front; their placement decides which defect patterns can be repaired.
Explained in Wafer-scale and SRAM-heavy designs (Architectures).
See also: Yield, Wafer-scale integration.