A placebo is a fake or inactive intervention, received by the participants allocated to the control group in a clinical trial, which is indistinguishable from the active intervention received by patients in the experimental group.
One definition is that use of a placebo describes what happens when you do nothing, so that in the context of a clinical trial, for instance, a placebo group could describe the natural history of a disorder without the intervention under test. It is the effect we see when we do nothing.
