quin·tes·sence / kwinˈtesəns/ • n. the most perfect or typical example of a quality or class: he was the quintessence of political professionalism. ∎  the aspect of something regarded as the intrinsic and central constituent of its character: we were all brought up to believe that advertising is the quintessence of marketing. ∎  a refined essence or extract of a substance. ∎  (in classical and medieval philosophy) a fifth substance in addition to the four elements, thought to compose the heavenly bodies and to be latent in all things.

quintessence (in classical and medieval philosophy) a fifth substance in addition to the four elements, thought to compose the heavenly bodies and to be latent in all things; it was believed that this essence could be extracted by alchemy.

From the late 16th century, quintessence has also meant the most perfect or typical example of a quality or class, as in the title of George Bernard Shaw's The Quintessence of Ibsenism (1891).

The word comes (in late Middle English) via French from medieval Latin quinta essentia ‘fifth essence’.

quintessence substance latent in all things, the extraction of which was one of the objects of alchemy XV; most essential part XVI. — F. quintessence, †quinte essence — medL. quinta essentia ‘fifth essence’, the fifth primary body besides the elements of earth, fire, air, and water.
Hence quintessential XVII.