The Collaborative International Dictionary of English v.0.48:

Almagest \Al"ma*gest\, n. [F. almageste, LL. almageste, Ar.
   al-majist[imac], fr. Gr. ? (sc. ?), the greatest
   composition.]
   The celebrated work of Ptolemy of Alexandria, which contains
   nearly all that is known of the astronomical observations and
   theories of the ancients. The name was extended to other
   similar works.
   [1913 Webster]