Swap space
Disk space the operating system uses as an "extension" of RAM — rarely used memory pages are moved out to disk (swapped) to free physical memory. It saves the system on memory shortage, but disk is orders of magnitude slower than RAM.
Mechanic: under memory pressure the kernel picks "cold" pages and writes them to swap (a partition or file); they're fetched back when needed. Win: programs don't crash immediately when RAM runs out, and idle pages can be parked to give more cache to active work. Trade-off: if the system actively swaps working memory, "thrashing" occurs → everything becomes extremely sluggish. On Linux swappiness controls how eagerly the kernel swaps. When both RAM and swap run out, the OOM killer steps in and kills processes. Windows equivalent: the pagefile. Modern variant: zram (compressed swap in RAM). Related to RAM disk and zram.