Veridium was a substance with a variety of uses, primarily to manufacture explosives. It had a half-life of 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 gave the Delta Flyer II a fuel converter rigged to leak veridium isotopes into the warp core. She intended for the resulting warp core breach to decimate those attending the Antarian Trans-stellar Rally. (VOY: "Drive")