In this section, we explore the fundamental concept of heat, defining it as a form of energy that moves naturally from bodies at higher temperatures to those at lower temperatures. We introduce key terminology and measurements, the SI unit of heat being the Joule (J), with the calorie (cal) as an alternative where 1 calorie equals 4.18 Joules. We delve into temperature, the average kinetic energy of particles within a substance, and discuss three common temperature scales: Celsius, Fahrenheit, and Kelvin, including conversions between them. Heat measurement is explained through the formula Q=mcΔT, where Q is the heat energy, m is the mass, c is the specific heat capacity, and ΔT is the change in temperature. The notion of specific heat capacity is introduced, emphasizing its importance in determining how different materials respond to heating. We also cover latent heat, important for phase changes during melting and boiling without temperature change. The section details modes of heat transfer: conduction, convection, and radiation, along with practical applications like thermometers and calorimetry. We conclude with a brief on heat engines and the laws of thermodynamics, grounding the theoretical aspects in real-world applications.