An essential oil is a concentrated, hydrophobic liquid containing volatile aroma compounds from plants. They are also known as volatile or ethereal oils, or simply as the "oil of" the plant material from which they were extracted, such as oil of ambrette. An oil is "essential" in the sense that it carries a distinctive scent, or essence, of the plant.

-- from Wikipedia

For centuries, oil has been extracted from plants by distillation in order to preserve their fragrance. Ancient Egyptian and Chinese texts document the properties of these precious substances, and Biblical references to their value abound.

-- from "Essential Oils: Distilled Oil is Used in Aromatherapy and Many Crafts" by S. Elliot