Glossary
Essential oil
The volatile aromatic compounds extracted from a plant by steam distillation, cold-pressing, or solvent — the concentrated essence of the plant's smell.
What makes basil smell of basil is a cluster of volatile molecules — eugenol, linalool, methyl chavicol — sitting in tiny oil glands on the leaf surface. Essential oil is the technical term for these compounds when isolated from plant matter. They are not fats; they are aromatic chemicals that evaporate at room temperature, which is why they smell.
In the kitchen, essential oils are why fresh herbs differ so much from dried ones (volatiles evaporate during drying), and why herbs added at the end of cooking taste different from herbs simmered for an hour (heat drives the volatiles off). The cooking sequence is essentially essential-oil management.