Veridium was a substance with a variety of uses, primarily to manufacture explosives. It had a half-life of less than seventy hours.
The cloaking devices used by the Ba'Neth left traces of veridium isotopes. In 2376, Naroq was able to use his photolitic converter to illuminate them while he was on the Delta Flyer, allowing a first glimpse into the morphology of a Ba'Neth. (VOY: "Riddles")
In 2377, Irina's ship utilized a veridium injector. Irina later gave the Delta Flyer a fuel converter rigged to leak veridium isotopes into the warp core, where the veridium reacted with the warp plasma. She intended for the resulting warp core breach to decimate those attending the Antarian Trans-stellar Rally. (VOY: "Drive")