Terminology
The name comes from the classical elements of the ancient Greeks. The aether, a pure "fifth element" (quinta essentia in Latin), was thought to fill the Universe beyond Earth. This seemed fitting to modern scientists, since quintessence was the fifth known contribution to the overall mass-energy content of the Universe. (The other four in the modern interpretation, different from the Greeks' ideas, are: baryonic matter; radiation – photons and the highly relativistic neutrinos, which may be considered hot dark matter; cold dark matter; and the term due to spatial curvature – loosely, gravitational self-energy.)
Read more about this topic: Quintessence (physics)