Heat transfer is the study of generation, use, conversion and exchange of thermal energy and heat between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation and transfer of energy by phase changes. The field of engineering also considers the transfer of mas of differing chemical species to achieve heat transfer. Although these mechanisms have distinct characteristics, they often occur simultaneously in the same system.
Heat is defined as the transfer of thermal energy across a well-defined boundary around a thermodynamic system. It is a characteristic of a process and is never contained in matter. However, the term heat transfer has acquired a specific usage. Heat is taken as synonymous to thermal energy. This can be interpreted by the heat of a fluid that can be transferred by various causes and that is also common in the language of laymen and everyday life.