The Free On-line Dictionary of Computing (30 December 2018):
A hypothetical disease the existence of which has
been deduced from the observation that unused programs or
features will often stop working after sufficient time has
passed, even if "nothing has changed". The theory explains
that bits decay as if they were radioactive. As time passes,
the contents of a file or the code in a program will become
People with a physics background tend to prefer the variant
"bit decay" for the analogy with particle decay.
There actually are physical processes that produce such
effects (alpha particles generated by trace radionuclides in
ceramic chip packages, for example, can change the contents of
a computer memory unpredictably, and various kinds of subtle
media failures can corrupt files in mass storage), but they
are quite rare (and computers are built with error detection
circuitry to compensate for them). The notion long favoured
among hackers that cosmic rays are among the causes of such
events turns out to be a myth.
Bit rot is the notional cause of software rot.
See also computron, quantum bogodynamics.