In chemistry, reflux is a process or condition that involves the continual vaporization and re-condensation of a liquid in a more-or-less closed system. Reflux setups are used to hasten some chemical processes that require heating without losing a solvent or liquid reagent used in the process. It can also be used for reactions that take place between two or more reactants in the gas phase. Finally, the physical state of reflux, in addition to those chemical processes, can also be used to perform extractions such as the production of essential oils from plant matter. For a guide on constructing reflux setups, see here.