Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to an object to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K).
Specific heat capacity of water
In thermodynamics, heat is the thermal energy transferred between systems due to a temperature difference. In colloquial use, heat sometimes refers to thermal energy itself. Thermal energy is the kinetic energy of vibrating and colliding atoms in a substance.
A glowing-hot metal bar showing incandescence, the emission of light due to its temperature, is often recognized as a source of heat
Rudolf Clausius
A red-hot iron rod from which heat transfer to the surrounding environment will be primarily through radiation.
Joseph Black