In medieval philosophy, the fifth essence or *element (after air, fire, water, earth) of which the celestial elements and regions were thought to be composed; sometimes thought to be an *elixir which cured all ills; latterly the most essential or purest part or quality of a person or thing. [< Lat. quinta = fifth + essentia = essence]
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.