In thermodynamics, the term exothermic process (exo- : "outside") describes a process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e.g. a battery), or sound (e.g. explosion heard when burning hydrogen). Its etymology stems from the Greek prefix (ex?, which means "outwards") and the Greek word (thermik?s, which means "thermal"). The term exothermic was first coined by Marcellin Berthelot. The opposite of an exothermic process is an endothermic process, one that absorbs energy usually in the form of heat.
Exothermic (and endothermic) describe two types of chemical reactions or systems found in nature, as follows.
Simply stated, after an exothermic reaction, more energy has been released to the surroundings than was absorbed to initiate and maintain the reaction. An example would be the burning of a candle, wherein the sum of calories produced by combustion (found by looking at radiant heating of the surroundings and visible light produced, including the increase in temperature of the fuel (wax) itself, which oxygen converts to hot CO2 and water vapor) exceeds the number of calories absorbed initially in lighting the flame and in the flame maintaining itself (some energy is reabsorbed and used in melting, then vaporizing the wax, etc. but is far outstripped by the energy released in converting the relatively weak double bond of oxygen  to the stronger bonds in CO2 and H2O).
On the other hand, in an endothermic reaction or system, energy is taken from the surroundings in the course of the reaction, usually driven by a favorable entropy increase in the system. An example of an endothermic reaction is a first aid cold pack, in which the reaction of two chemicals, or dissolving of one in another, requires calories from the surroundings, and the reaction cools the pouch and surroundings by absorbing heat from them. The production of wood by photosynthesis is an endothermic process: trees absorb radiant energy from the sun and use it in endothermic reactions such as taking apart CO2 and H2O and recombining the atoms to produce cellulose and other organic chemicals, as well as O2. The wood may later be burned in a fireplace, exothermically releasing the energy of O2 in the form of heat and light to their surroundings, e.g. to a home's interior.
Exothermic refers to a transformation in which a closed system releases energy (heat) to the surroundings, expressed by
When the transformation occurs at constant pressure and without exchange of electrical energy, heat Q is equal to the enthalpy change, i.e.
while at constant volume, according to the first law of thermodynamics it equals internal energy change, i.e.
In exothermic chemical reactions, the heat that is released by the reaction takes the form of electromagnetic energy or kinetic energy of molecules. The transition of electrons from one quantum energy level to another causes light to be released. This light is equivalent in energy to the stabilization energy of the energy for the chemical reaction, i.e. the bond energy. This light that is released can be absorbed by other molecules in solution to give rise to molecular translations and rotations, which gives rise to the classical understanding of heat. In an exothermic reaction, the energy needed to start the reaction is less than the energy that is subsequently released, so there is a net release of energy.
Some examples of exothermic processes are:
Chemical exothermic reactions are generally more spontaneous than their counterparts, endothermic reactions.
In a thermochemical reaction that is exothermic, the heat may be listed among the products of the reaction.
Because of historical accident, students encounter a source of possible confusion between the terminology of physics and biology. Whereas the thermodynamic terms "exothermic" and "endothermic" respectively refer to processes that give out heat energy and processes that absorb heat energy, in biology the sense is effectively inverted. The metabolic terms "ectothermic" and "endothermic" respectively refer to organisms that rely largely on external heat to achieve a full working temperature, and to organisms that produce heat from within as a major factor in controlling their bodily temperature.