An exothermic reaction is one in which heat is released from the reaction system to its surroundings, causing an increase in temperature. 'Exo' means to give off, heat is expelled in this reaction. An endothermic reaction is the opposite of this, instead heat is absorbed causing a decrease in temperature. 'Endo' mean to absorb. The difference in temperature change between the two reactions means a thermometer can be used to determine if a reaction is exothermic or endothermic.