A turbulent storm of charged particles encountered in space, dangerous to starships and their systems and blamed for several accidents in the 23rd and 24th centuries.
Ion storms recur throughout the franchise as a navigational hazard. In the Original Series episode Court Martial the Enterprise rides out a severe one, and Kirk is tried for jettisoning the ion pod with Benjamin Finney apparently still inside it.
In Mirror, Mirror an ion storm over the Halkan homeworld disturbs the transporter and exchanges Kirk's landing party with their counterparts from the mirror universe. Later series use the storms in much the same way, as a ready cause of crashed shuttlecraft, failed transporters and ships thrown off course.
Ships normally go around a storm when they can, since passing through one strains the hull and disrupts sensors, transporters and communications.