An essential oil is a concentrated non-water-based liquid containing volatile chemical compounds from plants. Essential oils are also known as volatile oils or simply as the oil of the plant from which they were extracted, such as oil of clove.
Essential oils are not the same as extracts. Typically, essential oils are made by distillation or pressing, which extracts the volatile organic compounds from the flesh, rind, bark, or leaves of a plant. Extracts are made by soaking plant material in a solvent, typically alcohol, water, or glycerin. As a result, extracts contain a broader range of compounds, including non-volatile ones, making the extract less concentrated, but often richer in the full range of the plant's ingredients.
An essential oil is essential in the sense that it contains the essence of the plant's fragrance — the characteristic fragrance of the plant from which it is derived. The term "essential" used here does not mean indispensable or usable by the human body, as with the terms essential amino acid or essential fatty acid, which are so called because they are nutritionally required by a living organism.