quintessenceyoudaoicibaDictYouDict[quintessence 词源字典]
quintessence: [15] Just as modern particle physicists search for the ultimate constituent of matter, the common denominator of all known forces, so medieval alchemists tried to find a fifth primary essence, which together with earth, air, fire, and water formed the substance of all heaven and earth. This fifth essence, higher and more ethereal than the other four, was postulated by Aristotle, who called it aithēr ‘either’.

Another Greek term for it was pemptē ousíā ‘fifth essence’, which was translated into medieval Latin as quinta essentia – whence, via French, English quintessence. The metaphorical sense ‘most perfect or characteristic embodiment’ began to emerge in the second half of the 16th century. Other English words based on quintus ‘fifth’, the ordinal form of Latin quinque ‘five’, include quintet [19] and quintuple [16].
[quintessence etymology, quintessence origin, 英语词源]
quintessence (n.)youdaoicibaDictYouDict
early 15c., in ancient and medieval philosophy, "pure essence, substance of which the heavenly bodies are composed," literally "fifth essence," from Middle French quinte essence (14c.), from Medieval Latin quinta essentia, from Latin quinta, fem. of quintus "fifth" (see quinque-) + essentia (see Parousia).

A loan-translation of Greek pempte ousia, the "ether" added by Aristotle to the four known elements (water, earth, fire, air) and said to permeate all things. Its extraction was one of the chief goals of alchemy. Sense of "purest essence" (of a situation, character, etc.) is first recorded 1580s.