A placebo is a medication that elicits a positive emotional response in an experimental subject?

A placebo is used in an experiment as a control. For example, when testing a new medication, experimenters will split the group in two- a control group and an experimental group. The experimental group receives the actual medication, while the control group receives a placebo (in this case, a sugar pill). The placebo will not have any actual effect on the person, but is meant to make sure people aren't making themselves believe that they are cured.