The CHEMICAL SPECIES that REACTS WITH THE ENDOGENOUS TARGET MOLECULE or CRITICALLY ALTERS THE BIOLOGICAL ENVIRONMENT, initiating structural and/or functional ALTERATIONS that result in toxicity. It CAN BE: - can be the ORIGINAL CHEMICAL to which the organism is exposed to (Parent Compound) - can be the a metabolite or a REACTIVE OXYGEN OR NITROGEN SPECIES (ROS or RNS) during the BIOTRANSFORMATION of the toxicant - can be ENDOGENOUS MOLECULE