something necessarily belonging to a thing, from which it cannot be conceived distinct: thus the primary qualities of bodies, as extension, figure, number, &c. are essential or inseparable from them in all their changes and alterations.

ESSENTIAL Oils are such as are really contained in a plant, and are drawn from it by distillation in an alembic with water: they are thus called, in contradistinction to empyreumatic oils, which are raised by a naked fire without water.