"An experimentally determined relation that shows that the dependent variable depends on or is a function of the independent variable and nothing else" (Johnston & Pennypacker, 2009, p. 358).

“A 'cause' becomes a 'change in an independent variable' and an 'effect' a 'change in a dependent variable.' The old 'cause-and-effect connection' becomes a 'functional relation.' The new terms do not suggest how a cause causes its effect; they merely assert that different events tend to occur together in a certain order" (Skinner, 1953, p.23).