An essential oil is a concentrated hydrophobic liquid containing volatile chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the oil of the plant from which they were extracted, such as oil of clove.
Essential oils are basically plant extracts. They're made by steaming or pressing various parts of a plant (flowers, bark, leaves or fruit) to capture the compounds that produce fragrance. It can take several pounds of a plant to produce a single bottle of essential oil.