LD50 (Lethal Dose 50): The LD50 is the dose of a substance that causes death in 50% of the population or test subjects.
TD50 (Toxic Dose 50): The TD50 is the dose of a substance that produces toxic effects in 50% of the population or test subjects.
EC50 (Effective Concentration 50): Similar to ED50, but refers to the concentration rather than dose, usually used for substances present in solutions.