Placebo

Any therapeutic procedure which has an effect on a patient, symptom, syndrome or disease, but which is objectively without specific activity for the condition being treated'. A medically or chemically inert substance or procedure that a person (usually participant or the researchers) believes will help them or make them recover. Placebos are used in scientific research to determine the effectiveness of the drug or treatment.

Reference:

De Craen, A. J., Kaptchuk, T. J., Tijssen, J. G., & Kleijnen, J. (1999). Placebos and placebo effects in medicine: Historical overview. Journal of the Royal Society of Medicine, 92(10), 511-515. https://doi.org/10.1177/014107689909201005