A union mount is a mount

Mount (computing)

Mounting takes place before a computer can use any kind of storage device . The user or their operating system must make it accessible through the computer's file system. A user can access only files on mounted media.- Mount point :A mount point is a physical location in the partition used as a...

 that allows several filesystems to be mounted at one time, appearing to be one filesystem.

Rather than mounting each filesystem at a different place in the directory hierarchy, a union mount overlays the filesystems, creating a unified hierarchy. Thus, any given directory (or "folder") in the resulting filesystem may contain files and subdirectories from any or all of the underlying filesystems.

Generally one of the filesystems will be mounted read-write, while other filesystems are mounted read-only.

Union mounts are implemented by a union filesystem such as UnionFS

UnionFS

UnionFS is a filesystem service for Linux, FreeBSD and NetBSD which implements a union mount for other file systems. It allows files and directories of separate file systems, known as branches, to be transparently overlaid, forming a single coherent file system...

 and AUFS

Aufs

aufs implements a union mount for Linux file systems.Developed by Junjiro Okajima in 2006 aufs is a complete rewriting of the earlier UnionFS...

. They originated with Plan 9 and its concept of union directories.