The physics and chemistry education literature has grappled with an appropriate definition for the concept of heat for the past four decades. Most of the literature promotes the view that heat is "energy in transit" or "involves the transfer of energy" between the system and surroundings because of a difference in temperature. Given that many undergraduate students are not learning the concept of heat in physics and chemistry alone, the goal of this investigation is to explore the conceptions of heat as presented in textbooks from across the science disciplines. An analysis of the definitions of heat from physics, chemistry, the biological sciences and the earth sciences showed a significant variation in the definitions within a discipline and between the disciplines. Specifically, the physics and chemistry textbooks used "energy in transit" or "transfer of energy" definitions (Class I), whereas textbooks from other disciplines typically used definitions which relate heat to "molecular kinetic energy" (Class II) or they used a hybrid of Class I and II definitions. Although a universal definition of heat across disciplines may not be possible (or even desirable), we suggest that discrepancies in definitions be acknowledged and clearly communicated to students. (Contains 1 table, 2 figures, and 5 notes.)