According to ancient and medieval science aether (Greek aithēr), also spelled æther or ether, is the material that fills the region of the universe

Universe

The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

 above the terrestrial sphere

Sublunary sphere

The sublunary sphere is a concept derived from Greek astronomy. It is the region of the cosmos from the Earth to the Moon, consisting of the four classical elements: earth, water, air, and fire. Beginning with the Moon, up to the limits of the universe, everything is made of aether...

.

Mythological origins

The word (aithēr) in Homeric Greek

Homeric Greek

Homeric Greek is the form of the Greek language that was used by Homer in the Iliad and Odyssey. It is an archaic version of Ionic Greek, with admixtures from certain other dialects, such as Aeolic Greek. It later served as the basis of Epic Greek, the language of epic poetry, typically in...

 means "pure, fresh air" or "clear sky", imagined in Greek mythology

Greek mythology

Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

 to be the pure essence where the gods lived and which they breathed, analogous to the air breathed by mortals (also personified as a deity, Aether

Aether (mythology)

Aether , in Greek mythology, is one of the Protogenoi, the first-born elementals. He is the personification of the upper sky, space, and heaven, and is the elemental god of the "Bright, Glowing, Upper Air." He is the pure upper air that the gods breathe, as opposed to the normal air that mortals...

, the son of Erebus

Erebus

In Greek mythology, Erebus , also Erebos , was often conceived as a primordial deity, representing the personification of darkness; for instance, Hesiod's Theogony places him as the first five beings to come into existence from Chaos...

 and Nyx). It is related to "to incinerate", also intransitive "to burn, to shine" (related is the name Aithiopes (Ethiopia

Ethiopia

Ethiopia , officially known as the Federal Democratic Republic of Ethiopia, is a country located in the Horn of Africa. It is the second-most populous nation in Africa, with over 82 million inhabitants, and the tenth-largest by area, occupying 1,100,000 km2...

ns)), meaning "people with a burnt (black) visage". See also Empyrean

Empyrean

Empyrean, from the Medieval Latin empyreus, an adaptation of the Ancient Greek ἔμπυρος empyrus "in or on the fire ", properly Empyrean Heaven, is the place in the highest heaven, which in ancient cosmologies was supposed to be occupied by the element of fire .-Use in literature:The Empyrean was...

.

Fifth element

In Plato

Plato

Plato , was a Classical Greek philosopher, mathematician, student of Socrates, writer of philosophical dialogues, and founder of the Academy in Athens, the first institution of higher learning in the Western world. Along with his mentor, Socrates, and his student, Aristotle, Plato helped to lay the...

's Timaeus

Timaeus (dialogue)

Timaeus is one of Plato's dialogues, mostly in the form of a long monologue given by the title character, written circa 360 BC. The work puts forward speculation on the nature of the physical world and human beings. It is followed by the dialogue Critias.Speakers of the dialogue are Socrates,...

 (St-55c) Plato described aether as that which God used in the delineation of the universe.
Aristotle

Aristotle

Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

 (Plato's student at the Akademia) included aether in the system of the classical element

Classical element

Many philosophies and worldviews have a set of classical elements believed to reflect the simplest essential parts and principles of which anything consists or upon which the constitution and fundamental powers of anything are based. Most frequently, classical elements refer to ancient beliefs...

s of Ionian philosophy

Greek philosophy

Ancient Greek philosophy arose in the 6th century BCE and continued through the Hellenistic period, at which point Ancient Greece was incorporated in the Roman Empire...

 as the "fifth element" (the quintessence), on the principle that the four terrestrial elements were subject to change and moved naturally in straight lines while no change had been observed in the celestial regions and the heavenly bodies moved in circles. In Aristotle's system aether had no qualities (was neither hot, cold, wet, or dry), was incapable of change (with the exception of change of place), and by its nature moved in circles, and had no contrary, or unnatural, motion (to be clearer, Aether did not have any properties: it was neither hot, cold, wet, or dry. It's natural motion was circular, and it does not have a contrary motion. Also, aether was supposed to be the heavens. Medieval scholastic philosophers granted aether changes of density, in which the bodies of the planets were considered to be more dense than the medium which filled the rest of the universe. Robert Fludd

Robert Fludd

Robert Fludd, also known as Robertus de Fluctibus was a prominent English Paracelsian physician, astrologer, mathematician, cosmologist, Qabalist, Rosicrucian apologist...

 stated that the aether was of the character that it was "subtler than light". Fludd cites the 3rd century view of Plotinus

Plotinus

Plotinus was a major philosopher of the ancient world. In his system of theory there are the three principles: the One, the Intellect, and the Soul. His teacher was Ammonius Saccas and he is of the Platonic tradition...

, concerning the aether as penetrative and non-material. See also Arche

Arche

Arche is a Greek word with primary senses 'beginning', 'origin' or 'first cause' and 'power', 'sovereignty', 'domination' as extended meanings. This list is extended to 'ultimate underlying substance' and 'ultimate undemonstrable principle'...

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Legacy

While special relativity

Special relativity

Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

 showed that Maxwell's equations do not require the aether, there were some early modern aether theories

Aether theories

Aether theories in early modern physics proposed the existence of a medium, the aether , a space-filling substance or field, thought to be necessary as a transmission medium for the propagation of electromagnetic waves...

, however, the early modern aether has little in common with the aether of classical elements from which the name was borrowed and the aether theories are scientifically obsolete.