Essential oils are highly concentrated aromatic extracts which are distilled from a variety of aromatic plant material including grasses, leaves, flowers, needles & twigs, peel of fruit, wood and roots.

The International Organization for Standardization (ISO) in their Vocabulary of Natural Materials (ISO/D1S9235.2) defines an essential oil as follows: "An essential oil is a product made by distillation with either water or steam or by mechanical processing of citrus rinds or by dry distillation of natural materials. Following the distillation, the essential oil is physically separated from the water phase."

Dr. Brian Lawrence states "For an essential oil to be a true essential oil, it must be isolated by physical means only. The physical methods used are distillation (steam, steam/water and water) or expression (also known as cold pressing, a unique feature for citrus peel oils). There is one other method of oil isolation specific to a very limited number of essential oil plants. This is a maceration/distillation. In the process, the plant material is macerated in warm water to release the enzyme-bound essential oil. Examples of oils produced by maceration are onion, garlic, wintergreen, bitter almond, etc. What is NOT an Essential oil is a CO2 extract, a halohydrocarbon extract or an empyreumatic distillate."