Which term describes an attribute or characteristic that does not differ across different subjects?

The term that describes an attribute or characteristic that does not differ across different subjects is a constant. A constant remains the same, regardless of the context or the subjects being observed in a study. For example, if every subject in a research study shares the same characteristic, such as age or a specific trait, that characteristic would be considered a constant for the purpose of that study.

In contrast, a variable refers to attributes or characteristics that can change and differ among subjects, such as income, educational level, or health status. Demographic variables specifically pertain to characteristics that define populations, like age, gender, or ethnicity, and these can also vary across individuals. Extraneous variables are those that are not the primary focus of research but could influence the outcome if not controlled; they are often overlooked in studies. Thus, a constant is distinguished by its unchanging nature across different subjects, making it the correct choice in this context.