(covers information from several alternate timelines)
An ion storm (also called an ionic storm or ionic front) is a type of magnetic storm which contains ionically charged particles, traveling at thousands of kilometers an hour. Both planetary and spaceborne ion disturbances can intensify to the point where navigation is dangerous, and transporter use all but impossible. At higher levels ion storms can damage or destroy spacecraft.
Ion storms were prevalent in the Denorios belt and had been since at least the 22nd century. In the Bajoran year 9174, Akorem Laan's lightship was caught in an ion storm there and was severely damaged. (DS9: "Explorers", "Accession")
The Kalandra star system had many ion storms. In 2375, Captain Benjamin Sisko and General Martok expressed their concerns the storms would reveal the Federation Alliance starships to the Dominion. (DS9: "The Emperor's New Cloak")
In the 23rd century, Constitution-class starships were equipped with an ion pod, allowing the starships to acquire data in conditions that defeated their conventional sensor platforms. Starships traversing ion storms were buffeted by the storms "weather" effects and subjected to tremendous hull pressures and stress. Starfleet used the measurement of a storm's relative effect on the ship as a supplementary data point to judge the intensity of an ion storm, and hazards it posed. Minor but noticeable "natural vibrations" were labeled Force-2 on an incremental scale. Above Force-7, the starship was at increased risk and subject to the captain's call for red alert status. (TOS: "Court Martial")
Later that year, an ion storm near the Halkan homeworld resulted in a power surge in the Enterprise's transporter, causing momentary interdimensional contact with a parallel universe. Captain Kirk, Doctor McCoy, Commander Scott, and Lieutenant Uhura, who were beaming up to the Enterprise at the time, materialized in the other universe, transposing with their counterparts from that universe, who experienced an identical accident at the same time. Later, after reviewing the events which led up to the accident, the Enterprise crewmembers were able to recreate the power surge using energy tapped from the ship's engines, and return to their own universe. (TOS: "Mirror, Mirror")
In late-2371, when the USS Defiant was preparing to enter the Bajoran Wormhole against orders from Admiral Toddman, the Admiral contacted them and repeated his orders to not enter the Gamma Quadrant. Captain Benjamin Sisko and Major Kira Nerys pretended that his transmission was being garbled by an ion storm, and proceeded anyway. (DS9: "The Die is Cast")