**2.1 Definition**

Essential oils (EO) are complex mixtures of volatile, lipophilic, and liquid compounds, extracted from different parts of a plant by physical processes. These interesting natural products have the characteristic odor of the plant.

According to ISO norm (ISO/D159235) "An essential oil is a product made by distillation with either water or steam or by mechanical processing of citrus rinds or by dry distillation of natural materiel. Following the distillation, the essential oil is physically separated from the water phase."

Essential oils are highly concentrated substances and therefore rarely used neat [1], they evaporate slowly when exposed to air at room temperature, and because of this, they are sometimes referred to as volatile oils [1].

EO's quality depends on many factors:

