Controlling for a variable refers to the deliberate varying of the experimental conditions in order to see the impact of a specific variable when predicting the outcome variable (see independent and dependent variables). Controlling tends to reduce the experimental error. A control is something that does not change in the experiment.
Controlling for a variable is also a term used in statistical data analysis when inferences may need to be made for the relationships within one set of variables given that some of the inter-relations may derive from relations to variables in another set. This is broadly equivalent to conditioning on the variables in the second set, although only linear relations may be taken into account. Such analyses may be described as "controlling for variable X", or "controlling for the variations in X".