The cosmos (/ˈkɒzmɒs/, US also /-moʊs, -məs/; Ancient Greek: κόσμος, romanized: kósmos) is an alternative name for the universe or its nature or order. Usage of the word cosmos implies viewing the universe as a complex and orderly system or entity.

The cosmos is studied in cosmology – a broad discipline covering scientific, religious or philosophical aspects of the cosmos and its nature. Religious and philosophical approaches may include the cosmos among spiritual entities or other matters deemed to exist outside the physical universe.

The verb κοσμεῖν kosmeîn meant generally 'to dispose, prepare', but especially 'to order and arrange' (troops for battle), 'to set (an army) in array'; also 'to establish' (a government or regime), 'to adorn, dress' (especially of women). Thus kósmos meant 'ornaments, decoration' (compare kosmokómes 'dressing the hair' and the English word cosmetic). The philosopher Pythagoras used the term kósmos for the order of the universe. Anaxagoras further introduced the concept of a Cosmic Mind (nous) ordering all things.

The 1870 book Dictionary of Greek and Roman Biography and Mythology noted

The book The Works of Aristotle (1908, p. 80 Fragments) mentioned

Bertrand Russell (1947) noted

Anaximander was a pre-Socratic Greek philosopher who is widely referred to as the "father of astronomy" and even as the "father of cosmology" as a result of his works to explain the origin and makeup of the physical universe. He is regarded as the most important of the Ionian philosophers, and was a pupil of Thales. Traditionally, details of his life and opinions are perpetuated not only by Aristotle and Theophrastos, but also by a great number of secondary authors. He lived throughout the fifth and fourth centuries, BCE, and was most likely the first philosopher to try to rationalize the system of the Earth, Sun, and Moon by the use of geometry and mathematics. Anaximander was also said to have created the first map of the world, however, like much of the rest of his works, this has been lost since his time. There is, however, documentation of Anaximander being responsible for the conception of the first mechanical model of the world, which is outlined by a geocentric model. He postulated that the Earth was at the center of the universe, and that its shape was convex and cylindrical, with life existing on one of the two flat sides. Beyond the Earth, sits the other planets, which Anaximander also details the order of. Next are the fixed stars, which he regarded as wheel-like condensations of air filled with fire, provided at certain places with openings through which flames are discharged. Anaximander places the Moon beyond these stars, and assumed it to also be wheel-like in shape, being nineteen times the size of Earth. Finally, on the top of the universe is the Sun, which interacts with the Moon, and the relationship between them is described in terms of aperture, in which a stoppage in would lead to eclipses.

In this model, the Sun is a ring, 28 times the size of the Earth, with a hollow rim, filled with fire, which at a certain place is seen through an aperture as in a pair of bellows. He also postulated regarding the formation of thunder and lightning, maintaining that they are caused by the wind becoming compressed inside a thick cloud and suddenly breaking through, causing the loud sound to be heard as the cloud is bursting. He claimed the fissure then looked like a spark because of the contrast with the dark cloud. Anaximander's model set a precedent for succeeding theories, including Copernicus's system, with the major change being the shift away from the geocentric model and towards the heliocentric model of the universe. The explained model, although accredited to Anaximander, did necessarily take from ideas originated in foreign cultures, such as the astronomical wheels which are known from Persian cosmology. But even without detailed commentary, these elements of the Anaximander tradition give a strong impression of an original and courageous thinker making conscious efforts towards producing a rational explanation of fundamental physical principles, the nature and motion of heavenly bodies, the shape of Earth, its place in the universe, etc.