Behavior is studied in several disciplines including psychology, ethology, and neuroscience. However, the definition of behavior has never been consistent not only across disciplines, but also between researchers within a field. The present article reviews this inconsistency and confusion both in the modern research fields and historical literatures. We find that the definitions of behavior are closely tied to the system of the fields, and the inconsistency of definitions rises from the differences of fields. As a result, the complete definition of behavior seems challenging. Instead of seeking for the complete definition, it is more productive to identify what makes definitions of behavior inconsistent. The article, thus, proposes three points to reduce the confusion accompanied by the inconsistency of the definitions: (1) stipulation of an organism as a whole and environment (2) internal-external distinction, and (3) formulation of the relationships between an organism and environment.