LD can mean: In science and mathematics: Lethal dose#, where "#" represents the percentage of test organisms killed by a specific dosage (of drugs, radiation, etc.): Half will die at LD50. The logarithm of base 2, mathrm(x)=log_2(x), from the Latin logarithmus dualis. Lunar distance, the distance between the Earth and the Moon and a measure of navigation. Linkage disequilibrium, when seemingly independent alleles have different rates of appearance (thanks to fitness concerns, etc.) in genetic… (More on Ld)