Webster's Dictionary defines "placebo" as "a medication prescribed more for the mental relief of a patient than for the actual effect on his disorder, or something tending to soothe."

The doctors tell us that if we know we are being treated with a placebo, it does not work. In our minds we must think that it is a real medication and has the strength or power to heal. If the patient believes this, then the treatment has been known to work wonders in many cases that otherwise could not have been treated. Placebo treatment is, in fact, nothing of substance, but in the mind of the patient it is real. In order for this kind of treatment to work, the doctor must convince the patient of the work of the medication.

 
http://www.spiritlessons.com/