Editing Quintessence (physics) (section)

== Terminology ==
The name comes from ''quinta essentia'' (fifth element). So called in Latin starting from the Middle Ages, this was the (first) element added by [[Aristotle]] to the other four ancient [[Classical element#Classical elements in Greece|classical elements]] because he thought it was the essence of the celestial world. Aristotle posited it to be a pure, fine, and [[Fortuna|primigenial]] element which he referred to as [[Aether (classical element)|''aether'']] in his text [[On the Heavens]]. Similarly, modern quintessence would be the fifth known "dynamical, time-dependent, and spatially inhomogeneous" contribution to the overall mass–energy content of the universe.

Of course, the other four components are not the [[Classical element#Classical elements in Greece|ancient Greek classical elements]], but rather "[[Baryonic matter|baryons]], [[neutrinos]], [[dark matter]], [and] [[Electromagnetic radiation|radiation]]." Although neutrinos are sometimes considered radiation, the term "radiation" in this context is only used to refer to massless [[photons]]. Spatial curvature of the cosmos (which has not been detected) is excluded because it is non-dynamical and homogeneous; the cosmological constant would not be considered a fifth component in this sense, because it is non-dynamical, homogeneous, and time-independent.<ref name=CDS />