What's the definition of placebo? Was always sure it's used as a control when testing medical treatments in which the patient doesn't know if they are receiving

What's the definition of placebo? Was always sure it's used as a control when testing medical treatments in which the patient doesn't know if they are receiving the treatment or not, leading to a genuine change in state from the belief they're in the experimental group, from which clinical significance is measured from as a baseline. I'm saying that psychological healing can have physiological effects above and beyond placebo due to the very real influence psychological state has on hormones. If you believe that it's a placebo effect, I'm not sure if it can be called a placebo effect? The whole point of a placebo is that you don't know it's a placebo otherwise it'd have stopped working long ago. Perhaps the common thread is the belief in its efficacy.