The description of the word you requested from the astronomical dictionary is given below.
The Almagest is the name by which the best-known astronomical book by Claudius Ptolemy is known since about the 10th century. This book was written by Ptolemy in the 2nd century in Greek (in Alexandria in Egypt) and was then called μαθηματική σύνταξις (Mathematike Syntaxis = Mathematical Treatise). It contains a complete description of a geocentric model for all planets then known and for the Sun and Moon, with explanations for how to calculate their positions and phenomena. The book became the standard book for astronomy, so it was later called Hè Megalè Syntaxis (The Great Treatise). In the 9th century, it was translated by Islamic astronomers into Arabic. The Greek version was forgotten in Western Europe. In the 12th century, Gerard of Cremona translated an Arabic version, which was then called al-mjsṭy "the greatest", into Latin, and so the book became known in Western Europe again, but now by the Latinized version of its Arabic title, the Almagest.