The thermodynamic quantity transferred because of a temperature difference is heat.
In thermodynamics, heat is energy transferred across a system boundary because the system and its surroundings are at different temperatures. It naturally flows from the hotter region toward the colder one unless external work or another mechanism drives the transfer in reverse. The SI unit for heat is the joule, and the usual symbol is Q.
Heat is not a substance stored inside an object. A system possesses internal energy, which includes microscopic kinetic and potential contributions, but “heat” describes energy in transit. Once energy crosses the boundary, it can change the system’s internal energy, temperature, phase, or other state properties.
This is commonly confused with work. Both heat and work are modes of energy transfer, but work occurs through organized forces or generalized displacements, while heat transfer is driven by temperature difference. Volume and mass are state variables, not quantities transferred specifically for that reason. Calorimetry measures heat indirectly through effects such as temperature change or melting ice.