Aristotle in his work On the Heavens gives the three meanings of the term 'ouranos'. According to the great philosopher, ouranos is, firstly, the outer sphere in his geocentric model, secondly, the body that occupies the next place to the outermost circumference of the world and thirdly, the Universe as a whole. The whole of the heavens, the whole cosmos, is spherical and moves continuously, and Aristotle (On the Cosmos B, 391b, p. 10) uses the term cosmos as a synonym for ouranos. This sense is quite common from Plato onwards. Additionally, we describe the meaning of the term 'aether', since 'aether', the fifth element (quintessence), is very important now in cosmological research.