吸热反应是吸收热量的一类化学反应，与放热反应相对。在吸热反应中，破坏化学键所用的能量大于组成键所释放的能量，其通式为：

反应物 + 能量 → 产物

因此，其焓变（ΔH）大于0。

• 绝大多数的分解反应
• 碱与铵盐生成氨气的反应
• 硝酸铵、硝酸钾(ΔH=34.8kJ/mol)溶解于水

The term endothermic process describes a process or reaction in which the system absorbs energy from its surroundings; usually, but not always, in the form of heat. The term was coined by Marcellin Berthelot from the Greek roots endo-, derived from the word "endon" (ἔνδον) meaning "within" and the root "therm" (θερμ-) meaning "hot." The intended sense is that of a reaction that depends on absorbing heat if it is to proceed. The opposite of an endothermic process is an exothermic process, one that releases, "gives out" energy in the form of (usually, but not always) heat. Thus in each term (endothermic & exothermic) the prefix refers to where heat goes as the reaction occurs, though in reality it only refers to where the energy goes, without necessarily being in the form of heat.