A thunderstorm (also called an electrical storm) is a type of storm which discharges electricity to a planet's surface. This storm typically includes rain. (VOY: "Cathexis")
It takes its name from thunder, the noise generated by the lightning in the storm.
Kathryn Janeway remarked to Chakotay in 2377 that there was no spatial anomaly scarier than a thunderstorm on the plains. When she was six, she saw a lightning bolt split an oak on her grandfather's yard, which she had climbed just hours before. (VOY: "Shattered")
When she was twelve, Janeway once walked home over seven kilometers in a thunderstorm because she had lost a tennis match. (VOY: "Deadlock")
After Felisa Howard's death, the Caldos colony had a thunderstorm due to a malfunction with its weather control system. (TNG: "Sub Rosa")
The Sky Spirits were capable of generating massive electrical storms on command, which they used to discourage unwanted visitors from their planet. (VOY: "Tattoo")