The Collaborative International Dictionary of English v.0.48:

Magnate \Mag"nate\, [F. magnat, L. (pl.) magnates, magnati, fr.
   magnus great. See Master.]
   1. A person of rank; a noble or grandee; a person of
      influence or distinction in any sphere; -- used mostly of
      prominent business executives; as, an industrial magnate.
      --Macaulay.
      [1913 Webster +PJC]

   2. One of the nobility, or certain high officers of state
      belonging to the noble estate in the national
      representation of Hungary, and formerly of Poland.
      [1913 Webster]

WordNet (r) 3.0 (2006):

magnate
    n 1: a very wealthy or powerful businessman; "an oil baron"
         [syn: baron, big businessman, business leader,
         king, magnate, mogul, power, top executive,
         tycoon]