Heat is defined as the transfer of thermal energy between systems due to a temperature gradient. This energy transfer is governed by the laws of thermodynamics and can occur through three primary mechanisms: conduction, which involves the direct transfer of kinetic energy between adjacent particles; convection, which requires a fluid (liquid or gas) to transfer heat by moving warmer fluid upward and cooler fluid downward; and radiation, which involves the transmission of energy via electromagnetic waves. The understanding and measurement of heat are fundamental to physical science, influencing fields from material science and engine design to biological processes like thermoregulation. The rate and method of heat transfer are crucial determinants of equilibrium and energy efficiency within closed systems.