A placebo is a treatment for a disease or condition which is deliberately ineffective. The motive usually is that if a person believes that a medicine, diet, or other treatment is good for them, then it is good for them.

Sometimes sick people who receive a placebo feel like they are getting better, and sometimes their bodies actually do get better. When patients have that response, it is called the "placebo effect". The term placebo effect (or placebo response) was introduced in 1920. It is the response of the subject which causes the observed effect, not the substance,.