Brewer's: Quintessence

The fifth essence. The ancient Greeks said there are four elements or forms in which matter can exist- fire, or the imponderable form; air, or the gaseous form; water, or the liquid form; and earth, or the solid form. The Pythagoreans added a fifth, which they called ether, more subtile and pure than fire, and possessed of an orbicular motion. This element, which flew upwards at creation, and out of which the stars were made, was called the fifth essence; quintessence therefore means the most subtile extract of a body that can be procured. It is quite an error to suppose that the word means an essence five times distilled, and that the term came from the alchemists. Horace speaks of “kisses which Venus has imbued with the quintessence of her own nectar.”

Swift to their several quarters hasted then The cumbrous elements- earth, flood, air, fire; But this ethereal quintessence of heaven Flew upward ... and turned to stars Numberless as thou seest.

Milton: Paradise Lost, iii. 716.

Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894

Quintilians

Quinsy