Currently, there is no agreement among scientists and science educators on whether heat should be defined as a "process of energy transfer" or "form of energy." For example, students may conceive of heat as "molecular kinetic energy," but the interpretation of this alternative conception is dependent on educational researchers' methodology and definition of heat. The objective of this paper is to review the alternative conceptions of heat as presented in empirical studies and to examine the possible sources of this subjectivity or causes of this problem. This paper analyzes the alternative conceptions of heat based on five categories: "residing in object," "ontological category," "movement," "cause and effect," and "condition." The findings suggest that it could be difficult to understand the alternative conceptions when there is disagreement on the definition or description of heat. Furthermore, alternative conceptions may be traceable to linguistic usage or definitions in textbooks. Therefore, it is possible to have a misinterpretation of students' alternative conceptions.