Wikipedia
OverlayFS
In computing, OverlayFS, a filesystem service for Linux, implements a union mount for other file systems.
It was merged into the Linux kernel mainline in 2014, in kernel version 3.18.
The main mechanics of OverlayFS relate to the merging of directory access when both filesystems present a directory for the same name. Otherwise, OverlayFS presents the object, if any, yielded by one or the other, with the "upper" filesystem taking precedence. Unlike some other overlay filesystems, the directory subtrees being merged by OverlayFS do not necessarily have to be from distinct filesystems.
OverlayFS supports whiteouts and opaque directories in the upper filesystem to allow file and directory deletion.