In thermodynamics, the term exothermic process (exo- : "outside") describes a process or reaction that releases energy from the system, 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 έξω (exō, which means "outwards") and the Greek word θερμικός (thermikόs, 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 in the form of heat.The concept is frequently applied in the physical sciences to chemical reactions, where as in chemical bond energy that will be converted to thermal energy (heat).Exothermic (and endothermic) are terms usually used to describe two types of chemical reactions or systems found in nature: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 increase in temperature of the fuel (wax) itself, which with oxygen, have become hot CO2 and water vapor,) exceeds the number of calories absorbed initially in lighting the flame and in the flame maintaining itself. (i.e. some energy produced by combustion is reabsorbed and used in melting, then vaporizing the wax, etc. but is far outstripped by the energy produced in breaking carbon-hydrogen bonds and combination of oxygen with the resulting carbon and hydrogen).Whereas, in an endothermic reaction, or system, energy is taken from the surroundings in the course of the reaction. An example of an endothermic reaction: a first aid cold pack, in which the reaction of two chemicals, or dissolving of one in another, requires calories from the surroundings, as the reaction cools the pouch and surroundings by absorbing heat from them. An endothermic system is seen in the production of wood: trees absorb radiant energy, from the sun, use it in endothermic reactions such as taking apart CO2 and H2O and combining the carbon and hydrogen generated to produce cellulose and other organic chemicals. These products, in the form of wood, say, may later be burned in a fireplace, exothermically, producing CO2 and water, and releasing energy in the form of heat and light to their surroundings, e.g., to a home's interior and chimney gasses.