The operational definition of a term is a detailed, specific and measurable definition of a term. It refers to the specific definition that you will use to measure the variable you are interested in. In other words, it is the way in which you will determine whether a subject has met the criteria for the variable under consideration.
Whenever data is collected, it should first be clearly defined what the data actually constitutes. Data that is not defined risks being inconsistent and might not give the same results when the study is repeated.
We often assume that those collecting the data know what they are doing and understand how to complete the task. However, different people may have different perspectives and interpretations of the same thing, and this will affect the data collection. The only way to ensure that the data is consistent is to provide a detailed operational definition of terms, which is usually found in the Methods section. This will standardise the data and help ensure future reliability. Aside from that, it’s simply good practise for any research paper to follow.