noun A synthetic estrogen drug C18H20O2 used formerly to prevent miscarriage or premature delivery in humans and to promote growth in livestock. The medical use of diethystilbestrol was discontinued in the U.S. during the early 1970s due to its association with serious side effects (such as reproductive tract abnormalities and increased rates of infertility) in offspring exposed to the drug while developing in the uterus. It is no longer commercially available in the U.S. and its use is currently limited to specially compounded formulations for veterinary care.