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For the notion of a controlled variable, see independent variable.

The term control variable has different meanings, depending on the area in which it is used.

in an experiment might be keeping the pressure constant in an experiment designed to test the effects of temperature on bacterial growth.

[edit] Control theory

In control theory, control 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 you only measure the effects of concentration. Variables that need to be controlled in this case include temperature, catalyst, surface area of solids, and pressure. If not controlled, they complicate the experiment and hence, the experiment is less valid.

 e.g. a control variable could be time or mass. E.g. each marble had ten minutes to dissove in acid. 

[edit] Programming

In programming, a control variable is a program variable that is used to regulate the flow of control of the program. For example, a loop control variable is used to regulate the number of times the body of a program loop is executed; it is incremented (or decremented when counting down) each time the loop body is executed.