Signal leakage is unwanted information escaping from a device through physical emissions such as radio-frequency radiation, electromagnetic interference, sound, light, or power variations. The leak is not part of the intended data path, but it can still carry enough structure for an attacker to infer what the device is doing.
In cyber security, signal leakage matters because it turns hardware behavior into a side channel. Attackers may use sensitive receivers, probes, or tuned interference to recover data from keyboards, displays, cables, headphones, and other electronics. Defenders reduce leakage with shielding, filtering, careful circuit design, device hardening, and controlled operating environments. The key idea is that even when software is secure, physical emissions can reveal secrets if the hardware creates a readable pattern.