A scientific investigation in which both the control group and experimental group(s) are kept under similar variables apart from the factor under study so that the effect or influence of that factor can be identified or determined.
A good example would be an experiment to test drug effects. The sample receiving the drug would be the experimental group while the sample receiving a placebo would be the control group. While all variables are kept similar (e.g. age, sex, etc.) the only difference between the groups is the taking of medication. If the control group and experimental group are showing similar results it means that the drug may be ineffective or is not showing significant therapeutic results.