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In psychology, behavior is what an organism does, described through actions or responses in a specified setting. A keypress, a spoken answer and a shift of gaze can all be measured, but they provide different evidence about the processes producing them. An operational definition states what will count as the response in a particular investigation.

For an original example, a learning task might measure how often a participant chooses a rewarded symbol and how long each choice takes. Choice frequency and reaction time are distinct outcomes: two participants can make equally accurate choices while taking different amounts of time. Neither measure alone establishes whether a difference arises from attention, comprehension, motivation or motor demands. The interpretation requires a model and suitable comparison conditions.

This distinction is central to construct validity: recording a response reliably does not by itself show that it measures the intended psychological construct. A report of an emotion is also a response to a question, rather than an unmediated measurement of every component of the emotion.

An operational definition specifies how a variable is measured or manipulated in a particular investigation. Defining stopping performance by a specified stop-signal task, scoring rule and estimation procedure makes the observation reproducible enough to inspect. The broader psychological construct of response inhibition need not be identical to that one procedure.^[ref]

For example, “number of correctly named digits from 1 to 10 under these instructions” operationally defines a numeral identification score. It leaves open whether the same child can compare quantities or count objects. An operational definition makes the measurement concrete; evidence about what that measurement represents is a separate question of construct validity.

A psychological construct is a theoretically specified attribute or process used to explain behavior and experience. Working memory, for example, is not directly observed as a number on an instrument; researchers infer aspects of it from performance under stated tasks. A psychological construct supplies an interpretation of observations, rather than merely renaming a test score.^[ref]

Different operational definitions can target the same proposed attribute. Their agreement, differences and relationships to other measures inform construct validity. If a memory task also requires reading, a low score might reflect reading difficulty as well as the intended memory demand. Clear theory and several complementary measurements help distinguish these explanations.