A "dummy" version of a drug (e.g. placebo pill). It's a common practice in medical experiments to separate a test group into two parts, giving some of the patients the real pill, while to the rest, a "placebo", a neutral pill containing no active ingredient, both designed to look the same. The patients aren't told which one of the pills they've got.

Such tests allow the researchers to separate the actual medical effects aside from possible mentally-caused effects (e.g. "side effects" or suddenly getting better from the placebo). Apparently, power of persuasion can do wonders to our health.