Hardware nonlinearity is the behavior of electronic components where the output does not change in a perfectly proportional way to the input. Instead of acting like a simple amplifier, filters, converters, analog stages, and even some power circuitry can mix, distort, or reshape signals. That distortion is often unwanted in engineering, but it can matter a lot in security.
In cyber security, nonlinearity can turn a weak external signal into a measurable change in emissions or other side channels. In TEMPEST-style scenarios, an attacker may inject a signal and rely on nonlinear parts of the target device to make hidden activity easier to observe. Defenders care because this means an air-gapped system can still leak information through electromagnetic paths. Shielding, careful component selection, placement, and emission testing help reduce that risk.