It is an inactive substance (pill, liquid, etc.), which is administered to a comparison group, as if it were therapy, but which has no therapeutic value other than to serve as a negative control.
It is an inert or innocuous substance used especially in controlled experiments testing the efficacy of another substance as a drug.
It is a pharmacologically inert preparation prescribed more for the mental relief of the patient than for its actual effect on a disorder during controlled experiments.
It is an inactive substance used in scientific studies. In clinical tests, new treatments are compared with placebo to make sure that any improvements are not the result of chance.