RAM disk
A virtual "disk" that lives entirely in RAM, giving extreme read/write speed but vanishing on reboot (volatile). Useful for temporary files, caches, and scratch data where speed is everything and persistence isn't needed.
Mechanic: the OS reserves a chunk of RAM and presents it as a filesystem. On Linux: tmpfs (where /tmp and /dev/shm often live) or ramfs; on Windows there are third-party tools. Win: orders of magnitude faster than even an NVMe SSD, perfect for compilation scratch, database temp files, video editing cache. Trade-off: everything disappears on reboot/power loss (volatile), and it eats into your working memory. Distinct from the OS's normal disk cache (which is automatic) by being an explicit, dedicated area. tmpfs can also swap out under memory pressure. A classic trick to speed up I/O-heavy, short-lived workflows. Related to swap and zram.