Al·ma·gest (?l?m?-j?st?)n.1. A comprehensive treatise on astronomy, geography, and mathematics compiled by Ptolemy about ad 150.2. almagest Any of several medieval treatises concerned with astronomy or alchemy.[Middle English almageste, from Old French, from Arabic al-majisti : al-, the + Greek megist? (suntaxis), greatest (composition), feminine of megistos, greatest, superlative of megas, great; see meg- in Indo-European roots.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.Almagest (?ælm??d??st) n1. (Astronomy) a work on astronomy compiled by Ptolemy in the 2nd century ad containing a description of the geocentric system of the universe and a star catalogue2. (Historical Terms) (sometimes not capital) any of various similar medieval treatises on astrology, astronomy, or alchemy[C14: from Old French, from Arabic al-majisti, from al the + majisti, from Greek megist? greatest (treatise)]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014Al?ma?gest (?æl m??d??st) n. 1. (italics) a Greek work on astronomy and mathematics by Ptolemy. 2. (l.c.) any of various similar treatises by medieval writers. [1350?1400; Middle English almageste