What aspect of research involves defining terms in measurable ways?

The concept of operational definition is crucial in research because it lays the groundwork for clarity and precision. An operational definition specifies the exact meaning of a variable by describing how it will be measured or identified in practical terms. For example, if a researcher wants to study "anxiety," they would need to define what anxiety means in their study—whether it be through questionnaires, physiological measurements, or observation of behavior.

By providing operational definitions, researchers ensure that their studies can be understood, replicated, and validated by others in the field. This is essential for maintaining rigor and reliability in scientific research. In contrast, the other options relate to different facets of research: procedure outlines methods, the theoretical framework connects theories guiding the research, and the research hypothesis poses a specific prediction. However, it is the operational definition that directly addresses how terms are quantified and measured, making it the correct choice for defining terms in measurable ways.