What defines an endothermic reaction?

An endothermic reaction is characterized by the absorption of heat from the surroundings. This means that during the course of the reaction, the reactants take in energy, typically in the form of heat, which leads to a decrease in temperature of the immediate environment. This energy absorption is essential for breaking bonds in the reactants and forming new bonds in the products.

In the context of the other choices, a reaction that releases heat pertains to exothermic processes, which is the opposite of an endothermic reaction. The production of gas does not specifically classify a reaction as either endothermic or exothermic, as gas can be produced in reactions of both types. Finally, the occurrence of a catalyst is unrelated to whether a reaction is endothermic or exothermic; catalysts simply speed up reactions without altering the heat characteristics. Thus, the defining feature of an endothermic reaction is the heat absorption, making it the correct choice.