Covariate

In statistics, a covariate is a variable that is possibly predictive of the outcome under study. A covariate may be of direct interest or be a confounding variable or effect modifier.

Also called covariable. An independent variable, or predictor, in a regression equation. Also, a secondary variable that can affect the relationship between the dependent variable and independent variables of primary interest in a regression equation

In econometrics, term "control variable" is usually used instead of covariate.

Reference

 * A Dictionary of Epidemiology: Fourth Edition. John M Last ed. Oxford UP 2001.