The US Centers for Disease Control and Prevention's Vaccines and Immunizations Glossary defines a placebo as "A substance or treatment that has no effect on human beings." [1]
A placebo is a substance or treatment used as a decoy in studies for the "control" part of the sample. Part of the study group will receive a placebo, and the remainder the group will receive the new medication being studied, so that the differences in outcome may be compared.
It is well known to medical researchers that merely assuring someone that a treatment will be successful, greatly increases the chance of the treatment actually working.[Citation Needed] For example, a tension headache will frequently disappear if the patient believes they are being given an analgesic.[Citation Needed] This is known as the placebo effect. It's effect is due entirely to the psychological effect of a patient knowing they are receiving a treatment.
The effectiveness of a pain relief medicine is always contrasted with the effectiveness of a placebo. The question is not how well it relieves pain, but how much better is it than a placebo?