bit rot: n. Also {bit decay}. 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

:bit rot: n. Also {bit decay}. 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
increasingly garbled.

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-detecting circuitry to compensate
for them). The notion long favored among hackers that cosmic
rays are among the causes of such events turns out to be a myth;
see the {cosmic rays} entry for details.

The term {software rot} is almost synonymous. Software rot is
the effect, bit rot the notional cause.
-- The AI Hackers Dictionary