Wiktionary
lethal dose
n. the dose of a substance or of radiation likely to cause death (sometimes suffixed with a number)
WordNet
lethal dose
n. the size dose that will cause death
Wikipedia
Lethal dose
In toxicology, the lethal dose (LD) is an indication of the lethality of a given substance or type of radiation. Because resistance varies from one individual to another, the "lethal dose" represents a dose (usually recorded as dose per kilogram of subject body weight) at which a given percentage of subjects will die. The lethal concentration is a lethal dose measurement used for gases or particulates. The LD may be based on the standard person concept, a theoretical individual that has perfectly "normal" characteristics, and thus not apply to all sub-populations.