(covers information from several alternate timelines)
Delta radiation is a highly energetic release of electromagnetic particles which can be extremely dangerous with prolonged exposure. Delta radiation can be created by the warp cores of Federation starships and some natural astronomical phenomena.
In 2153, Commander Tucker and Doctor Phlox determined that the organic power cells in Xindi-Reptilian rifles could be neutralized by delta radiation, raising the possibility of using portable EM emitters to disable the Xindi weapons. (ENT: "The Shipment")
In the mirror universe, Enterprise's warp core produced dangerous levels of delta radiation, causing noticeable disfigurement to Chief Engineer Charles Tucker. Tucker noted that, for every year of exposure to the warp core, one loses ten years off one's life expectancy. (ENT: "In a Mirror, Darkly")
In 2267, Fleet Captain Pike was conducting an inspection of a Class-J training vessel, when a ruptured baffle plate exposed a large number of cadets to delta radiation. Exposing himself to a dangerous level of the radiation in order to rescue the cadets, Pike was severely injured and left an invalid. (TOS: "The Menagerie, Part I")
The same year while on a rescue mission for the runabout USS Yangtzee Kiang, Jadzia Dax noted that the data from the long range sensors might be inaccurate because of an abnormal level of delta radiation from the star in the binary star system. (DS9: "Battle Lines")