A control variable is an element that is not changed throughout an experiment because its unchanging state allows the relationship between the other variables being tested to be better understood.
Be careful not to confuse the different types of variable in an experiment. A useful way to remember the difference is that your results depend on what you do, so they are your dependent variable. To make sure it is a fair test you need to control other things that could affect the result they are your controlled variables. Which only leaves the thing which you deliberately change that is your independent variable.