Essential oil is a volatile aromatic substance extracted from flowers, leaves, stems, roots or fruits of plants by steam distillation, extrusion, cold soaking or solvent extraction. Essential oils are divided into diluted (compound essential oils) and undiluted (unilateral essential oils). Essential oils are highly volatile and will evaporate quickly once they come into contact with air. For this reason, essential oils must be stored in sealable bottles, and once they are opened for use, they should be covered as soon as possible.