The cosmos (, ) is the universe. Cosmos is used at times when the universe is regarded as a complex and orderly system or entity; the opposite of chaos. The cosmos, and our understanding of the reasons for its existence and significance, are studied in cosmology, which is a very broad term covering any scientific, religious, or philosophical contemplation of the cosmos and its nature, or reasons for existing. Religious and philosophical approaches may include in their concept of the cosmos various spiritual entities or other matters deemed to exist outside our physical universe.
The philosopher Pythagoras first used the term cosmos (Ancient Greek: ) for the order of the universe. The term became part of modern language in the 19th century when geographer-polymath Alexander von Humboldt resurrected the use of the word from the ancient Greek, assigned it to his five-volume treatise, Kosmos, which influenced modern and somewhat holistic perception of the universe as one interacting entity.
Cosmology is the study of the cosmos, and in its broadest sense covers a variety of very different approaches: scientific, religious and philosophical. 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.
When cosmology is used without a qualifier, it often signifies physical cosmology, unless the context makes clear that a different meaning is intended.
Physical cosmology (often simply described as 'cosmology') is the scientific study of the universe, from the beginning of its physical existence. It includes speculative concepts such as a multiverse, when these are being discussed. In physical cosmology, the term cosmos is often used in a technical way, referring to a particular spacetime continuum within a (postulated) multiverse. Our particular cosmos, the observable universe, is generally capitalized as the Cosmos.
In physical cosmology, the uncapitalized term cosmic signifies a subject with a relationship to the universe, such as 'cosmic time' (time since the Big Bang), 'cosmic rays' (high energy particles or radiation detected from space), and 'cosmic microwave background' (microwave radiation detectable from all directions in space).
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)
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 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