A form of energy postulated to be causing the expansion of the universe to accelerate; i.e. the dark energy in the universe. Theories of quintessence do not assume that dark energy is described by a cosmological constant. No convincing theory of quintessence has yet emerged.
Astronomy
A proposed explanation for the force commonly known as dark energy that causes the apparent acceleration in the expansion of the Universe. Quintessence literally translates as ‘fifth essence’, a fifth fundamental force of nature that dominates over gravity on very large scales. Quintessence models typically predict a slow decrease in the density of dark energy as the Universe evolves, in contrast to the cosmological constant explanation for dark energy which maintains a constant density. This results in a different expansion history, which may be observable with future supernova surveys.
Philosophy
In Aristotle, the fifth element, distinct from earth, fire, air and water, exempt from change and decay, and found only in celestial bodies. More informally, the intrinsic or essential nature, or the perfect exemplar, of anything.