Term: VSS [Volume Snapshot Service]
VSS is an acronym for Volume Snapshot Service. Also known by other names, including Shadow Copy, VSS is a service in Microsoft Windows designed to allow back-up software to take backup copies of files, even though they may be in use and otherwise locked.
Traditionally, a file being used by a running program is locked so other programs cannot access the file at the same time. This most often applies to files that could be written to, since having the file change could cause other programs to get confused were they allowed to simultaneously attempt to access it.
Unfortunately, this type of locking prevents a traditional backup program from being able to back up the file.
To grossly oversimplify, VSS creates a snapshot of the file(s) in question at a point in time – a “shadow copy” – that can be safely backed up, even though the original file might continue to be locked, written to, or otherwise modified.