An operational definition, also called functional definition, defines something (e.g. a variable, term, or object) in terms of the specific process or set of validation tests used to determine its presence and quantity. That is, one defines something in terms of the operations that count as measuring it. The term was coined in philosophy of science book The Logic of Modern Physics (1927), by Percy Williams Bridgman, and is a part of the process of operationalization. One might use definitions that rely on operations in order to avoid the troubles associated with attempting to define things in terms of some intrinsic essence.
An example of an operational definition might be defining the weight of an object in terms of the numbers that appear when that object is placed on a weighing scale. The weight then, is whatever results from following the (weight) measurement procedure, which should be repeatable by anyone. This is in contrast to operationalization that uses theoretical definitions.
Read more about Operational Definition: Overview, Limitations, Application, Theoretical Vs Operational Definition
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