Which type of reaction is defined as one that releases energy in the form of heat?

The term defining a reaction that releases energy in the form of heat is exothermic reaction. In exothermic reactions, the energy released is often evident as an increase in temperature of the surroundings. These reactions occur when the total energy of the products is less than the total energy of the reactants, resulting in a net release of energy. A common example of an exothermic reaction is combustion, where substances react with oxygen, releasing heat and often light.

While combustion reactions are indeed a specific type of exothermic reaction characterized by the burning of a substance, the term exothermic encompasses all reactions that release heat, not just those involving combustion. Endothermic reactions, on the other hand, absorb heat (and thus do not release energy). Redox reactions involve the transfer of electrons between species and are not specifically defined by energy release; they can be either exothermic or endothermic depending on the specific substances and conditions involved.