An Etymological Dictionary of Astronomy and Astrophysics

In cosmology, a hypothetical new “element,” distinct from any normalmatter (either → baryonic or not) or radiation, intended to explain the observed ever → accelerating expansion of the Universe. Quintessence can have several types and differs from the → cosmological constant in that it can vary in space and time. In modern physics, the four known “elements” are the → baryons (proton, neutron, etc.), the → leptons (neutrinos, electrons, etc.), the → photon, and the hypothetical → non-baryonic matter, which is thought to be 80% of the total matter in the Universe.
The quintessence field is a possibility which can be confirmed or disproved by measurements of the → dark energy value at different → redshifts. In some models, the quintessence is fine-tuned to explain both the → cosmological constant problem and the
→ inflation in the very → early Universe.

Etymology (EN): Literally “fifth essence,” from M.Fr. quinte essence, from M.L. quinta essentia, from L. quinta, fem. of quintus “fifth,” from quinque “five,” cognate with Pers. panj, → five + essentia “being, essence,” from esse “being; → existence” + -entia “-ence.” In Aristotelianism, the fifth element, distinguished from the four earthly elements, was the substance of celestial bodies. Subsequently, quintessence became the purest, most highly concentrated form of a nature or essence.

Etymology (PE): Panjomin gowhar “fifth essence,” from panjomin “fifth,” from panj, → five, cognate with L. quinque, + gowhar “essence, substance; jewel, pearl, gem,” Mid.Pers. gohr “essence, substance; jewel; stock, lineage;” cf. Skt. gôtra- “family, race, lineage, origin.”