Abstract We define heat as a particular kind of nonwork interaction that involves only energy and entropy transfers, and that is entirely distinguishable from work. The existence of heat interactions is a consequence of the first and second laws of thermodynamics. The requirement that heat be entirely distinguishable from work implies strict conditions on the end states of the interacting systems, and guarantees a definite relation between such states and the energy and entropy transfers. We illustrate these conditions by using energy versus entropy graphs. Many experiences can be represented as heat interactions, including the exchanges between two black bodies at temperatures that differ infinitesimally. We discuss the latter point in a companion paper at this conference.