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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. Further, a control variable strongly influences values; it is held constant to test the relative impact of independent variables.
In scientific experimentation, a control variable is the one element that must not be changed throughout an experiment because it also affects the other independent variables being tested, thus affecting the outcome of the experiment. For example, in the experimental verification of Boyle's law (P = T / V), where Pressure, Temperature, and Volume are all variables; to test the resultant changes to any of these variables requires at least one of the other variables to be kept constant. This is in order to see comparable experimental results in the remaining variables. Essentially, a controlled variable is what is kept the same throughout the experiment. It's also a process of science.
Other candidates for controlled variables might be, for example, if you are testing a product's effects on two plants, the soil type and the pot shape may be two controlled variables. Duration of sunlight and amount of water might be others.
In control theory, controlled variables are the variables that are input into 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 the test is less valid. Without a control variable, you would not be able to compare results.