The LD50 (Lethal Dose 50) is the dosage of a chemical substance at which 50% of the sample population dies. This measurement is used primarily as a way of comparing toxicities of various substances (for instance, anesthetics). It is also used as a method of determining the maximum safe dosage.
It is important to emphasize, however, that this is a mortality statistic. At LD50, 50% of the sample population has died. It is entirely possible, and probable, that there have been deaths before LD50 was reached. As a result, LD50 dosages should be treated as the absolute upper bound, not as a safe upper bound.
LD50 is measured in mass of substance per mass of body mass (i.e. milligrams per kilogram), thereby giving the relative toxicity of a given chemical. An LD50 of 5mg/kg would be considered to be lower than 10mg/kg, thereby making the first substance the more potentially toxic of the two.