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) such as cactus seed oil. Essential oils are highly volatile and will evaporate quickly when exposed to air, so they must be kept in a dark bottle that can be sealed.

Not all plants can produce essential oil, only plants containing balsam glands can produce essential oil. Different plants have different distribution of balsam glands, some of which are on petals, leaves, roots or trunks. After extracting the sachet, it becomes what we call "plant essential oil". Essential oils contain many different components. Some essential oils, such as roses, can be composed of more than 250 different molecules. Essential oils are lipophilic and soluble in oil, because the molecular chain of essential oils is usually very short, which makes them easily penetrate into the skin and enter the body through rich capillaries under subcutaneous fat. Essential oils are made up of some very small molecules. These highly volatile substances can be absorbed by the nasal mucosa and enter the body, sending information directly to the brain, and regulating emotions and physical functions through the limbic system of the brain. Therefore, in aromatherapy, essential oil can strengthen physiological and psychological functions. Each plant essential oil has a chemical structure that determines its fragrance, color, fluidity and the way of interaction with the system, which also makes each plant essential oil have a set of special functional characteristics.