Level of dosage for a substance,
controlled or otherwise, which is
lethal for 50% of
subjects who take that amount. Data like this is (
obviously) usually only available for
non-humanoids.
For example, the LD50 of LSD is 46 mg/kg intravenously in mice -- compare to an active dosage of .001 mg/kg (1 microgram/kg) in humans.