Ionization disturbances were disturbances related to ionization, the process of turning an atom or molecule into an ion. Cloaking devices produced such disturbances.
In 2366, a Federation class-1 probe detected ionization disturbances in orbit of Nelvana III. Data considered it possible those were produced by cloaked Romulan ships. It later turned out that they were created by a cloaked orbital probe, which the Romulans claimed to use for archaeological research. (TNG: "The Defector")
When the planet Meridian suddenly appeared in the Trialus system in 2371 there was no residual ionization, so the crew of the USS Defiant ruled out the possibility of the planet having been cloaked. (DS9: "Meridian")