Essential oil refers to the general term for volatile aroma-containing substances obtained by processing and extracting from perfume plants or scent-secreting animals. Generally, essential oils are volatile aromatic substances extracted from plant flowers, leaves, roots, seeds, fruits, bark, resin, wood core and other parts through steam distillation, cold pressing, liposuction or solvent extraction. . Essential oils are divided into diluted (compound essential oils) and undiluted (single essential oils) such as cactus seed oil. Essential oils are very volatile and will evaporate quickly once they come into contact with air, so essential oils must be stored in dark, sealable bottles.

Not all plants can produce essential oils, only plants that contain balsam glands can produce essential oils. The distribution of balsam glands in different plants is different, some are on petals, leaves, rhizomes or tree trunks. After the sachet is refined and extracted, it becomes what we call "plant essential oil". Essential oils contain many different ingredients. Some essential oils, such as rose, can be formed by combining more than 250 different molecules. Essential oils are lipophilic and easily dissolve in oils, because the molecular chains of essential oils are usually relatively short, which makes them very easy to penetrate the skin and enter the body through the abundant capillaries under the subcutaneous fat. Essential oils are composed of some very small molecules. These highly volatile substances can be absorbed into the body by the nasal mucosal tissues and send information directly to the brain. Through the limbic system of the brain, it regulates emotions and physiological functions of the body. Therefore, in aromatherapy, essential oils can strengthen physical and psychological functions. Every plant essential oil has a chemical structure that determines its fragrance, color, fluidity, and the way it works with the system. It also makes each plant essential oil have a special set of functional characteristics.