Essential oils are highly volatile substances isolated by a physical process from an aromatic plant of a single botanical species. Such oils were called ‘essential’ because they were thought to represent the very essence of the odour and flavour of the plant. Aromatic plants include herbs, shrubs and trees of all sizes, and may be annual, biennial or perennial. The oil-accumulating organ or tissue varies between species and no single part of the plant is excluded from the list of organs which serve as oil stores.

An essential oil may contain up to several hundred chemical compounds and this complex mixture of compounds gives the oil its characteristic fragrance and flavour. They are isolated from plant parts by various methods such as distillation, solvent extraction, enfluerage (absorbing into fixed oils and fats), fractional distillation, head space techniques and extraction with liquid carbondioxide.