For an exothermic reaction at constant pressure, the system's change in enthalpy equals the energy released in the reaction, including the energy retained in the system and lost through expansion against its surroundings. In a similar manner, for an endothermic reaction, the system's change in enthalpy is equal to the energy "absorbed" in the reaction, including the energy "lost by" the system and "gained" from compression from its surroundings. If is positive, the reaction is endothermic, that is heat is absorbed by the system due to the products of the reaction having a greater enthalpy than the reactants. On the other hand, if is negative, the reaction is exothermic, that is the overall decrease in enthalpy is achieved by the generation of heat.