Write policy for maintaining coherency between cache and main memory. Write-back (also called copy back or store in) writes only to the block in local cache. Writes occur at the speed of cache memory. The modified cache block is written to main memory only when the corresponding memory address is referenced by another processor. The processor can write within a cache block multiple times and writes it to main memory only when referenced. Because every write does not go to memory, write-back reduces demands on bus bandwidth. See also cache, coherence, write-through.