The defining characteristic of an endothermic reaction is that it absorbs heat from its surroundings, resulting in a temperature decrease in the environment. During such reactions, the system takes in energy to facilitate the conversion of reactants into products, often requiring external energy input, such as heat. This property distinguishes endothermic reactions from exothermic ones, which release energy. Understanding endothermic reactions is essential for applications in fields like chemistry, thermodynamics, and even biological processes like photosynthesis.