Lethal dose
Lethal dose (LD) is an indication for the fatality of a given substance or radiation. It does not state a specific dose which kills an organism, but (because of varying resistance among different individual members of the species) a dosage at which a percentage of organisms dies.
This was often (and sometimes still is) used in drug research, where scientists would try to find the LD50 (50% of the animals die after the dosage is applied) for various animals (rats, bunnies, etc.).