Meaning of median lethal dose

me'dian le'thal dose'

Pronunciation: [key]
  1. the quantity of a lethal substance, as a poison or pathogen, or of ionizing radiation that will kill 50 percent of the organisms subjected to it in a specified time period. Symbol: LD
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.