The Collaborative International Dictionary of English v.0.48:

Generalissimo \Gen`er*al*is"si*mo\, n. [It., superl. of generale
   general. See General, a.]
   The chief commander of an army; especially, the commander in
   chief of an army consisting of two or more grand divisions
   under separate commanders; -- a title used in most foreign
   countries.
   [1913 Webster]

WordNet (r) 3.0 (2006):

generalissimo
    n 1: the officer who holds the supreme command; "in the U.S. the
         president is the commander in chief" [syn: commander in
         chief, generalissimo]