capacitive coupling

Hold your hand near a microphone cable plugged into a powerful amplifier and you will hear a buzz, without touching anything at all. Two pieces of metal sitting near each other form a tiny capacitor through the air between them, and a changing voltage on one pushes a little current onto the other through that invisible capacitor. That sneaky transfer of an unwanted signal through stray capacitance is capacitive coupling. It is how electric fields leak interference from one wire into a neighbouring one.

The coupling capacitance is small, perhaps a fraction of a picofarad between two nearby wires, but its reactance falls with frequency (Xc = 1/(2 times pi times f times C)), so the higher the frequency or the faster the edge, the more easily noise crosses. The injected noise also grows with the noisy wire's voltage swing and with the impedance of the victim node: a high-impedance input of a megohm or more is exquisitely vulnerable, while a low-impedance node shorts the tiny coupled current harmlessly to ground. That is exactly why a floating, high-impedance scope probe picks up mains hum the instant you wave it in the air.

The fixes attack each factor in turn: lower the victim's impedance, increase the separation (the capacitance falls as the wires move apart), and above all place a grounded conductor between source and victim, a shield or a ground trace, so the stray field terminates on ground instead of on your signal. Honest distinction to remember: capacitive coupling is driven by VOLTAGE and electric fields, and a grounded electrostatic shield stops it well. This is quite different from inductive coupling, which is driven by current and magnetic fields and needs a different cure entirely.