The term for energy that is expelled during a chemical reaction is called enthalpy. In exothermic reactions, energy is released as heat, leading to a decrease in the system's internal energy. This energy change can often be measured in joules or calories. Conversely, in endothermic reactions, energy is absorbed from the surroundings, resulting in a temperature drop. Understanding enthalpy is crucial for applications in areas such as thermodynamics, chemical engineering, and environmental science.