The first use of "cosmos" in English occurred in 1650 and of "cosmology" in 1656. The medieval Latin words that best describe what we understand by "cosmos" are mundus, caelum, and universum. The first, mundus, could have as many as four meanings but usually embraced at least the heavens and earth and all that lay between. The second, caelum, usually translated as "heaven" or "heavens," was more limited in scope. In its narrowest signification, it could represent a single planetary sphere (and is appropriately translated as "heaven"), but it could also signify the entire celestial region, which included the totality of celestial orbs, everything from the sphere of the Moon to the sphere of the fixed stars and whatever orbs might lie beyond. (In this context, caelum is better translated as "heavens," but in historical usage the terms "heaven" and "heavens" have usually been interchangeable, as they often are in this study.) And, as we shall see, caelum was even sometimes used for subdivisions of the world that excluded only the earth. The third term, universum, was probably synonymous with the broadest meanings of mundus but, in the definitions provided by Pierre d'Ailly, takes on special significance in this study. The first takes as its domain the totality of celestial bodies; the second equates universum with the prime mover, a definition that was not to be taken seriously; the third definition at first glance embraces everything as part of the universe, thus including the aggregate of celestial bodies, the intelligences that are applied to them, all the mixed bodies and the four elements contained under the Moon.