Variables can be classified into three categories: 1. Independent (input, manipulated, treatment, or stimulus) variables, so called because they are "independent" of the outcome; instead, they are presumed to cause, effect, or influence the outcome. 2. Dependent (output, outcome, response) variables, so called because they are "dependent" on the independent variable; the outcome presumably depends on how these input variables are managed or manipulated. 3. Control (background, classificatory, or organismic) variables, so called because they need to be controlled, held constant, or randomized so that their effects are neutralized, canceled out, or equated for all conditions.
US Environmental Protection Agency. (2007). Program Evaluation Glossary. Office of the Administrator/Office of Policy/Office of Strategic Environmental Management/Evaluation Support Division.