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The term control variable has different meanings, depending on the area/place in which it is used. The control variable is something that is constant and unchanged in an experiment.
A control variable is any factor that remains unchanged and strongly influences values.
In scientific experimentation, a control variable is the one that must not be changed throughout an experiment because it affects the independent variables and thus affects the outcome of the experiment. For example, in the experimental verification of Boyle's law, the temperature must be kept constant. Essentially, a controlled variable is what is kept the same throughout the experiment. An example of a controlled variable would be if you pancha have experimented on plants and tested a product on two plants, the soil and the pot would be two controlled variables.
In control theory, controlled variables are the variables that are input to the control system. Reaction rate is the dependent variable, and everything else that can change the reaction rate must be controlled (kept constant) so that the analyst can only measure the effects of concentration. Variables that need to be controlled in this case include the catalyst, the surface area of solids, and pressure. If not controlled, they complicate the experiment, and hence, the test is less valid. T'is used for your experiment to be more exact, because without a control variable you would not be able to compare results.