A lethal form of radiation produced as industrial waste by the Malon, who hauled it in vast freighters and dumped it in other people's space rather than adopt cleaner technology.
Theta radiation is a deadly contaminant created by the Malon's crude use of antimatter. Their civilisation produces billions of tonnes of the waste, and export crews carry it far from home and vent it wherever is convenient.
In the Voyager episode Night, the Malon hauler Emck is found dumping theta radiation into a starless void, poisoning the beings who live there. Janeway offers to share Starfleet methods of purifying reactor products, but Emck refuses because cleaner technology would put him out of business.
In Juggernaut, Voyager's crew board a crippled Malon freighter leaking theta radiation that threatens everything within light years, and learn how badly the work scars the core labourers who shovel the waste.