Economic Definition of variable input. Defined.
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Term variable input Definition: An input whose quantity can be changed in the time period under consideration. This should be immediately compared and contrasted with fixed input. The most common example of a variable input is labor. A variable input provides the extra inputs that a firm needs to expand short-run production. In contrast, a fixed input, like capital, provides the capacity constraint in production. As larger quantities of a variable input, like labor, are added to a fixed input like capital, the variable input becomes less productive. This is, by the way, the law of diminishing marginal returns.