placebo effect

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The beneficial effect in a patient following a particular treatment that arises from the patient's expectations concerning the treatment rather than from the treatment itself.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun pharmacology The tendency of any medication or treatment, even an inert or ineffective one, to exhibit results simply because the recipient believes that it will work.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• 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