INPUT: The resources or factors of production used in the production of a firm's output. This term is most frequently associated with the analysis of short-run production, and is often modified by the terms fixed and variable, as in fixed input and variable input. In the short run, the quantity of a fixed input can not be changed, meaning it can not be used to expand output. In contrast, a variable input can be changed, making it THE means of expanding output in the short run. See also | resources | factors of production | short-run production | fixed input | variable input | output | production | law of diminishing marginal returns | marginal product | average product | total product |