The cosmos (, ) is the universe regarded as a complex and orderly system; the opposite of chaos. The philosopher Pythagoras used the term cosmos (Ancient Greek: ) for the order of the universe, but the term was not part of modern language until the 19th century geographer and polymath, Alexander von Humboldt, resurrected the use of the word from the ancient Greek, assigned it to his multi-volume treatise, Kosmos, which influenced modern and somewhat holistic perception of the universe as one interacting entity.
Cosmology is the study of the cosmos in several of the above meanings, depending on context. All cosmologies have in common an attempt to understand the implicit order within the whole of being. In this way, most religions and philosophical systems have a cosmology.
Cosmology is a branch of metaphysics that deals with the nature of the universe, a theory or doctrine describing the natural order of the universe. The basic definition of Cosmology is the science of the origin and development of the universe. In modern astronomy the Big Bang theory is the dominant postulation.
In physical cosmology, the term cosmos is often used in a technical way, referring to a particular spacetime continuum within the (postulated) multiverse. Our particular cosmos, the observable universe, is generally capitalized as the Cosmos.
According to Charles Peter Mason in Sir William Smith Dictionary of Greek and Roman Biography and Mythology (1870, see book screenshot for full quote), Pythagoreans described the universe.
It appears, in fact, from this, as well as from the extant fragments, that the first book (from Philolaus) of the work contained a general account of the origin and arrangement of the universe. The second book appears to have been an exposition of the nature of numbers, which in the Pythagorean theory are the essence and source of all things. (p. 305)
In theology, the cosmos is the created heavenly bodies (sun, moon, planets, and fixed stars). In Christian theology, the word is also used synonymously with aion to refer to "worldly life" or "this world" or "this age" as opposed to the afterlife or world to come.
The 1870 book Dictionary of Greek and Roman Biography and Mythology noted
The book The Works of Aristotle (1908, p. 80 Fragments) mentioned