The gradual degradation of the bits on storage media that are not in constant use. There is no guarantee that an internal or external drive not accessed for three or more years will retain 100% of its data. Preventative solutions are writing the same data to two or more drives as well as continuous backup to an external service.
Data fade mostly occurs on hard drives, but solid state drives (SSDs) are vulnerable as well. Also called "data rot" and "bit rot," if SSD blocks or disk sectors are not rewritten periodically, the magnetic bits or charged cells may weaken and cause the data to be unreadable.
Magnetic tapes have a similar problem. Because tape is tightly coiled together, the magnetic bits begin to interfere with each other over time. When tape cartridges are archived, they must be recopied periodically. See
magnetic tape formats,
bit rot and
RAID.