What are essential oils?

Aromatherapy uses essential oils, but what exactly are essential oils?

Essential oils are obtained from flowers, stems, leaves, roots, sap and bark of specific plants and trees.

The amount of oil produced varies from plant to plant, and the chemical composition can change depending on time of day or season the plant is gathered.
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There are different ways of extracting the oils including maceration, enfleurage, and steam distillation.