In order to understand what the heat is, it is necessary to explain some introductory notions. The universe consists of matter. The matter exists in form of substance or physical field. Energy is a property of matter. There is movement because the energy exists. Energy travels faster or slower depending on the wave transmission method (the electric field, electromagnetic field, oscillating mechanical motion, etc.). The energy propagates through substance. The substance opposed to unidirectional motion of waves and it imprints an oscillatory movement to the particles that are component parts of the respective substance. These particles (that are part of the substance) can be: molecules, atoms, ions, or electrons). The oscillatory movement is applied to the particles in several directions. That energy is called “heat”.
So, heat is nothing but energy or in other words, the Joule effect is heat. The energy that faces opposition in its unidirectional propagation is called heat. Because of the heat, the particles of the substance have kinetic energy (energy of motion) and they oscillate. The notion of heat is also related to perception of living organisms that have sense organs for taking cognizance of the existence or absence of the heat. Heat propagates in three ways: through contact, through convection and through radiation.
The heat propagates through contact when two good thermal conductor bodies (such as two metal bars) have physical contact.
The heat spreads through convection with the help of substance as well, but in this case the substance has to be liquid or gas (fluids). The warm fluid becomes lighter (thermal agitation increases the distance between its molecules, so cohesion forces decrease) and they rise up ceding the space to cooler molecules that are heavier.
The heat propagates in straight line (both mechanical and electromagnetic wave) and it reflects on a shiny and opaque surface.
So, heat can only exist where there is substance and not in vacuum, because the particles of the substance have to oscillate in different directions. The vacuum means the absence of substance, but not the lack of physical fields. A wave can be viewed by throwing a stone into a pond. Note the circles formed where the stone fell. Potential energy of the stone was sent to water through few circular plane waves. The phenomenon of mechanical radiation reflection can be observed when a sound is made in a cave. The mechanical waves are reflected by the ice crystals.
So, in conclusion, what is heat? Heat is the energy that propagates through the mechanical waves in all directions and having the effect of asynchronous oscillatory motion of particles of substance. The most common measurement units for heat are the calorie and the joule. Heat is a scalar quantity. Joule is the unit of measurement for mechanical work as well. So here’s another proof that there is a very close relation between energy and movement.