APA Dictionary of Psychology

the minimum amount of a drug that is required to produce a specified effect. It is usually expressed in terms of median effective dose, or effective dose 50 (ED50, ED-50), the dose at which 50% of the nonhuman animal test population has a positive response. In psychotherapy, this criterion is also used to express the number of sessions that are needed for 50% of patients to show a clinically significant change. See also therapeutic ratio.