An exothermic process is one that gives off heat. This heat is transferred to the surroundings. An endothermic process is one in which heat has to be supplied to the system from the surroundings. A thermoneutral process is one that neither requires heat from the surroundings nor gives off energy to the surroundings. These terms are usually applied to chemical reactions. A chemical reaction can only be one of these three terms at once. A reaction that is exothermic will be endothermic if run backward and vice-versa.