overlay filesystem
also: overlayfs, overlay2
A filesystem that layers multiple directory trees on top of each other, combining them into a single unified view where upper layers can shadow or modify lower layers without changing the originals.
An overlay filesystem (overlayfs) allows you to merge two or more directory trees—called layers—into a single mount point. The upper layer can add new files or hide files from lower layers, while the lower layers remain untouched on disk.
This is particularly useful in containerization and live systems. For example, you might have a read-only base filesystem as the lower layer and a writable layer on top. When a container writes to a file that exists in the base layer, overlayfs automatically copies it to the writable layer (copy-on-write behavior), leaving the original unchanged.
Linux kernels 3.18+ include native overlayfs support. It powers Docker containers, Podman, and other container runtimes. A typical setup uses lowerdir (read-only base), upperdir (writable changes), and workdir (internal working directory) mounted at a mergedir junction point.