What is Lethal Dose 50 (LD50) in?

Question

What is Lethal Dose 50 (LD50) in?

Answer 1

LD50 is a means of measuring toxicity. Basically it means "Lethal Dose in 50% of the subjects"

If I were to take 100 rats that weighed 1kg(for simplicity) and give them 100mg of a substance then 50% of them died as a result, the LD50 would be 100mg/kg in rats. For most substances, LD50 is similar in creatures. There are a few exceptions.

For most substances, if you give less than the LD50, less than 50% would die, more and more than 50% would die. Again, this is an average over a population.  

Answer 2

The dose that on average would be expected to kill 1/2 of the individuals (people, rats, mice, whatever) in a population (of people, rats, mice, whatever).

There is also an LD 1, the dose that on average would kill one out of every 100 individuals who would take it.