English to Arabic Science Mathematics & Statistics

Controlling for a variable is a method to reduce the confounding effect of variations in a third variable that may also affect the value of the dependent variable. For example, in an experiment to determine the effect of nutrition (the independent variable) on organism growth (the dependent variable), the age of the organism (the third variable) needs to be controlled for, since the effect may also depend on the age of an individual organism.
http://en.wikipedia.org/wiki/Controlling_for_a_variable

Another definition
Control for a variable: To control for a variable is to try to separate its effect from the treatment effect, so it will not confound with the treatment. There are many methods that try to control for variables. Some are based on matching individuals between treatment and control; others use assumptions about the nature of the effects of the variables to try to model the effect mathematically, for example, using regression.
http://www.stat.berkeley.edu/~stark/SticiGui/Text/gloss.htm