Original: In toxicology, the median lethal dose, LD50 (abbreviation for “Lethal Dose, 50%”), LC50 (Lethal Concentration, 50%) or LCt50 (Lethal Concentration & Time) of a toxic substance or radiation is the dose required to kill half the members of a tested population.
Simplified: It is an abbreviation for "Lethal Dose, 50%" or median lethal dose.