A constant variable, normally called a controlled variable, is the term for a variable that remains constant throughout an experiment, though other variables may change. An example is the water pressure for a faucet when measuring the amount of water released when the faucet is opened to various increments.
Controlled variables are necessary in determining cause and effect through scientific experimentation. Experiments require that constants be maintained in order to monitor how changes such as increase of temperature or chemical reactions affect dependent or independent variables within the experiment. Controlled variables allow for accurate determinations to be made as long as the controlled variable remains constant.