'Placebo effect' definitions:

Definition of 'placebo effect'

noun

Any effect that seems to be a consequence of administering a placebo; the change is usually beneficial and is assumed result from the person's faith in the treatment or preconceptions about what the experimental drug was supposed to do; pharmacologists were the first to talk about placebo effects but now the idea has been generalized to many situations having nothing to do with drugs

Definition of 'Placebo effect'

From: GCIDE

Placebo effect \Pla*ce"bo ef*fect`\, n. (Med.) a reaction by a patient who receives a placebo[2], in which the symptoms of illness are lessened or an anticipated effect is experienced. Because the placebo[2] itself has no pharmacological activity, this reaction is mediated by the expectations of the patient receiving the placebo[2]; the reaction is considered as an example of the power of suggestion.
Note: Dramatic subjective effects such as relief of discomfort or pain are sometimes observed due to administration of a placebo, but in some cases measurable physiological effects may also be observed. [PJC]