WORM – Write Once Read Many – refers to storage media which can only be written to once. This makes it suitable for archives and reference information which cannot and should not be altered. Optical disk formats, such as CD, DVD and Blu-ray, had WORM variants. WORM media is inherently immutable. Its contents cannot be altered. Otherwise mutable storage media can be made virtually immutable by having retention policies, such as ‘never delete’; applied to them. For example AWS S3 storage ObjectLock. But this is virtual WORM technology, not physical, and therefore less dependable.