a "mock" intervention that is not thought to have a significant inherent effect. Examples of placebos include a pill that contains an inert substance (e.g. a sugar pill), an intravenous infusion of saline, or a sham surgery where an incision is made but no procedure performed. The goal when using a placebo in experimentation is to have the placebo resemble the actual treatment as much as possible without creating the effects of the intervention that are being investigated. In other words, the use of a placebo in experimentation allows the treatment and control groups to be alike in every respect except for the actual effects of treatment (i.e. both groups get a white pill, but only the pills given to the treatment group have an effect--the control group gets a placebo).
Learn more: The powerful influence of placebos on the brain