A term used to describe any storage medium that is non-volatile and whose data cannot be accessed by the computer once removed. A good example of offline storage is a USB thumb drive. Offline storage is used for transport as well as backup protection in the face of unpredictable events, such as hardware failure due to power outage or files corrupted by a computer viruses.
The image shows three types of storage, but offline storage is a subset of secondary storage as they both serve the same purpose and do not interact directly with the CPU. The key differences being that offline storage is used as a way to physically transport information, generally has less capacity, and isn't accessible without human interaction.