Heat is the thermal energy transit from one system to another. The thermal energy can originate from any kind of energy according to the first law of thermodynamics. Transfer of heat is due to unique property of matter, temperature, and is governed by second law of thermodynamics, which dictates that free flow of heat is possible only from a body of higher temperature to that at a lower temperature.
All heat transfer processes, therefore, involve the transfer of energy and obey the first as well as the second law of thermodynamics. The energy in transit cannot be measured or observed directly, but the effects it produces can be observed and measured. From our viewpoint, the determination of the rate of heat transfer needs special consideration.
The transport of heat energy from one region to another occurs by any (or a combination) of similar methods. In literature such three methods of heat transmission are recognized by the terms conduction, convection and radiation respectively.
If the flow of heat is a result of transfer of internal energy from one molecule to other, the process is called conduction. Through solids, this is the only possible mode of heat transmission. In liquid and gases, however, the molecules are no longer confined to a certain point but constantly change their positions even if the substance is at rest. The heat energy is transported along with the motion of these molecules from one region to another. This process is called convection. All solid bodies as well as liquids and gases have a tendency of radiating thermal energy in the form of electromagnetic waves and of absorbing similar energy emerging from the neighboring bodies. This type of heat transport is known as thermal radiation.
In industrial processes, heat transfer may occur due to one or due to a combination of more than one of these three modes of transport.