A substance which has no effects on the test subject, given to members of the control group during experimental trials that test the effects of a drug or other substance. The control group is the group which does not receive the experimental drug or substance and is used to compare what happens to the test group or groups. This is done because the number of variables between different groups is reduced by going through the same motions of giving a substance to everyone. Also, especially in double-blind experiments, placebos are used to minimize bias in interpreting the experimental results. It has the additional benefit that in trials with humans, some people's illnesses may improve if they simply believe that they've been given medicine, even if they've gotten sugar pills (a common placebo in drug trials) instead. This psychological phenomenon is called a "placebo effect."
Selected placebo links: