An operational definition is the complete description of the processes that will take place when performing an investigation. In easier terms, the steps that you are going to use to investigate, your action plan, needs to be described thoroughly, not only for purposes of validity and reliability, but also because it needs to be specific and show its relationship to the investigation. Someone who wants to repeat your experiment will want to know exactly how you did it. It is no different than repeating a recipe that you found on a website and needing the EXACT same ingredients that the original chef used, along with prep time, and how to serve it.
Sociology offers a myriad of opportunities for research and analysis of data. However, to analyze data you have to build valid and reliable research methods and models. All findings depend on the variables of the method, that is, on which factors are put under investigation, and how these factors interact when altered or left the same. This is why operational definitions find their weight. In quantitative and qualitative analysis the procedures must be exact, clean, and able to be repeated over and over. It is the scientific method, and its transparency and clarity must be evident. That is the importance behind these definitions.
In sociology, operational definitions translate an abstract concept into something that is measurable and observable in an experiment. For example, the operational definition of the concept of assimilation of foreign-born teenagers in the United States might be the degree to which they dress in typical western clothing, listen to American pop music, and practice American customs (such as going out alone with their friends instead of socializing with only their families). If a sociologist were studying foreign-born teenagers, the sociologist might use this operational definition of assimilation in his or her study or experiment, as the abstract concept of assimilation would not be useful or efficient for research purposes. The process by which sociologists turn abstract concepts into operational definitions is one of the most complicated and controversial parts of sociological studies. Operational definitions are also used in other social sciences, such as psychology.