Do you mean a control in an experiment?
If so, this is a side experiment with which you'll take reference, and to prove your hypothesis.
Say that you want to prove that plants release oxygen.
You'll set up an experiment, with an aquatic plant, with the required apparatus. Now, you'll collect a gas, which turns out to be oxygen.
However, one can say that the plant didn't produce that oxygen, but the dissolved air in the water for example. Having a control, with the same conditions except that there is no plant, and where no gas was evolved shows that only the plant could have produce that oxygen!
I hope it helped!
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