Heat or thermal conduction is a transport phenomenon. wherein energy transformations result in work done by systems or bodies that results in a lowering of the thermal energy level of the originating system and a concordant increase in the thermal energy level of the destination system or body.

Simply put, thermal energy lowers in a body of hotter gas, liquid or solid, and increases in regions with initially lower temperatures (an indirect measure of heat). The direction of the transportation of energy always takes place in the direction in which the temperature decreases. The means by which this occurs, however, will vary depending on whether the body is a gas, liquid or a solid.

The simplest model of thermal "transport" or heat "flow" takes place in a gas. In the kinetic molecular model concepts of heat and temperature directly reflect molecular velocities and rates of collisions...