In a quantitative scientific experiment, subjects are divided into two groups. The variable being tested is applied to one group, but not the other; the second group, which does not receive the treatment, is the control group.
Example: Scientists examining the effect of a new anti-cancer medication divide their subjects into two groups. One group receives the medication, while the control group gets a placebo.
Grinnell, R. (2008). Control Group. Psych Central. Retrieved on April 27, 2015, from http://psychcentral.com/encyclopedia/2008/control-group/