What is the X in X-mas?
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.