People have always understood that something flows from hot objects to cold ones. We refer to that as heat transfer.

     Heat transfer (or heat) is the thermal energy in transit due to a temperature difference.

     Whenever there exists a temperature difference in a medium or between media, heat transfer must occur. However, heat transfer need not always result in a change in the temperature; Eg.: when we supply heat to a boiling liquid, the temperature of boiling liquid does not increase, as the heat supplied (termed as latent heat of evaporation) is used for the change of phase- from liquid phase to vapour phase.
      Heat transfer is all-pervasive. This scientific understanding came as early as 19th century! Joseph Fourier (1768-1830) said that “Heat, like gravity, penetrates every substance of the universe; its rays occupy all parts of space. The theory of heat will hereafter form one of the most important branches of general physics.”
      A natural question arises. The question as well as the answer, both are given in what follows.