absorbing heat without increase in temperature when heated beyond a certain point. Antonyms: exothermal, exothermic, heat-releasing. (of a chemical reaction or compound) occurring or formed with the liberation of heat.In thermochemistry, an endothermic process is any process with an increase in the enthalpy H (or internal energy U) of the system. ... In such a process, a closed system usually absorbs thermal energy from its surroundings, which is heat transfer into the system.One example, which can be carried out in a lab, is the reaction between barium hydroxide octahydrate and ammonium nitrate. This reaction can be demonstrated by combining, in a 2:1 ratio, solid barium hydroxide octahydrate with dry ammonium nitrate in a small beaker at room temperature.