A is a substance or treatment that appears identical to the actual treatment but lacks the active substance.

The substance or treatment you're referring to is known as a "Placebo". A placebo is designed to mimic the actual treatment but does not contain an active substance or ingredient. It's often used in clinical trials as a control group to compare the effects of the actual drug. The patients in the placebo group believe they are receiving the actual treatment, which can sometimes lead to a "placebo effect" where patients report improvements in their condition due to their belief in the treatment rather than the treatment itself.