In research design, especially in psychology, social sciences, life sciences and physics, operationalization (or operationalisation) is a process of defining the measurement of a phenomenon which is not directly measurable, though its existence is inferred from other phenomena. Operationalization thus defines a fuzzy concept so as to make it clearly distinguishable, measurable, and understandable by empirical observation. In a broader sense, it defines the extension of a concept—describing what is and is not an instance of that concept. For example, in medicine, the phenomenon of health might be operationalized by one or more indicators like body mass index or tobacco smoking. As another example, in visual processing the presence of a certain object in the environment could be inferred by measuring specific features of the light it reflects. In these examples, the phenomena are difficult to directly observe and measure because they are general/abstract (as in the example of health) or they are latent (as in the example of the object). Operationalization helps infer the existence, and some elements of the extension, of the phenomena of interest by means of some observable and measurable effects they have.

Sometimes multiple or competing alternative operationalizations for the same phenomenon are available. Repeating the analysis with one operationalization after the other can determine whether the results are affected by different operationalizations. This is called checking robustness. If the results are (substantially) unchanged, the results are said to be robust against certain alternative operationalizations of the checked variables.

The concept of operationalization was first presented by the British physicist Norman Robert Campbell in his Physics: The Elements (Cambridge, 1920). This concept spread to humanities and social sciences. It remains in use in physics.

Operationalization is the scientific practice of operational definition, where even the most basic concepts are defined through the operations by which we measure them. The practice originated in the field of physics with the philosophy of science book The Logic of Modern Physics (1927), by Percy Williams Bridgman, whose methodological position is called "operationalism".

Bridgman wrote that in the theory of relativity a concept like "duration" can split into multiple different concepts. In refining a physical theory, it may be discovered that what was thought to be one concept is actually two or more distinct concepts. Bridgman proposed that if only operationally defined concepts are used, this will never happen.

Bridgman's theory was criticized because "length" is measured in various ways (e.g. it is impossible to use a measuring rod to measure the distance to the Moon), so "length" logically is not one concept but many, with some concepts requiring knowledge of geometry.^{[citation needed]} Each concept is to be defined by the measuring operation used. So the criticism is that there are potentially infinite concepts, each defined by the methods that measured it, such as angle of sighting, day of the solar year, angular subtense of the moon, etc. which were gathered together, some astronomical observations taken over a period of thousands of years.

In the 1930s, Harvard experimental psychologist Edwin Boring and students Stanley Smith Stevens and Douglas McGregor, struggling with the methodological and epistemological problems of defining measurement of psychological phenomena, found a solution in reformulating psychological concepts operationally, as it had been proposed in the field of physics by Bridgman, their Harvard colleague. This resulted in a series of articles that were published by Stevens and McGregor from 1935, that were widely discussed in the field of psychology and led to the Symposium on operationism in 1945, to which Bridgman also contributed.

The practical 'operational definition' is generally understood as relating to the theoretical definitions that describe reality through the use of theory.