Almagest

Almagest or the Almagest is the name of the Arabic version of Ptolemy's treaties on astronomy.

However, in the Middle Ages, the word almagest referred to any greatest composition, since the Arabic word al means the and the Greek word megiste means 'greatest (composition)'. These two words entered into French as almageste and then came into English as almagest. It is because of this any celebrated textbook on astrology or alchemy were called almagest in the Middle Ages.