A heatwave was a period of excessively hot weather which may have been accompanied by high humidity.
Andoria experienced a heatwave every few years. (ENT: "The Aenar")
In 2373, the USS Voyager's main power was failing and environmental controls were going offline, with systems shutting down one by one. Captain Kathryn Janeway told Neelix to expect a heatwave once environmental controls failed because heat from the warp plasma conduits couldn't be vented. Neelix said he was used to it as he grew up near the Rinax marshlands where the summers were the hottest in the sector. Fifty degrees Celsius, ninety percent humidity, and the most vicious lavaflies. Janeway claimed summers in Indiana were similar when she was growing up. (VOY: "Macrocosm")