An essential oil is that aromatic, volatile liquid that is within many shrubs, flowers, trees, roots, bushes, and seeds, and that is usually extracted through steam distillation.
Essential oils are chemically complex, and may be made up of hundreds of chemical compounds. They are highly concentrated and far more potent than dried herbs because of method of distillation.
5,000 pounds of rose petals = 1 kilo of rose oil
They are different from vegetable oils, such as canola or olive oil, in that they do not go rancid (if properly distilled) and are powerful antimicrobials. The method for producing the oils determines their purity. Many oils readily available, especially lavender, have been produced with high heat or pressure, or have their oils extracted with solvents. These oils are adulterated, and it is often difficult to tell except through analytical testing.