A scientific control is an experiment or observation designed to minimize the effects of variables other than the variable that is tested for. This increases the reliability of the results, often through a comparison between control measurements and the other measurements. Scientific controls are a part of the scientific method.
The most common types of control are negative and positive controls. These two controls are usually sufficient to eliminate the risk of unknown variables.
Ideally, all variables in an experiment will be controlled (accounted for by the control measurements) and none will be uncontrolled. In such an experiment, if all the controls work as expected, it is possible to conclude that the experiment is working as intended and that the results of the experiment are due to the effect of the variable being tested.