noun, plural: volatile oils
Volatile oils are oils that are characterized by their volatility and failure to saponify. They evaporate when they are exposed to the air and thus are capable of distillation. They are derived from plant tissues. Thus, they may be produced naturally by extraction, particularly by distillation, often by using steam. They may also be made synthetically.
As medicines, volatile oils are used as stimulants, stomach aches, correctives, and carminatives. Other use of volatile oil is for purposes of flavoring (e.g., peppermint oil). But the most common use of volatile oil is for the production of perfumes, cosmetics, soaps, and the like.
Volatile oil is sometimes called an essential oil. An essential oil is known for being a concentrated hydrophobic liquid with plant-derived, volatile aroma compounds. It is referred to as essential since it contains what seems to be the essence of the plant's fragrance.1
1"essential oil". Oxford English Dictionary
(online, American English ed.).