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]

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "magnate":
   Brahman, Establishment, VIP, archduke, aristocrat, armiger, banker,
   baron, baronet, big boss, big businessman, big gun, big man,
   big name, big wheel, big-timer, bigwig, blue blood, brass,
   brass hat, business leader, businessman, captain of industry,
   celebrity, count, czar, daimio, dignitary, dignity, director, duke,
   earl, elder, enterpriser, entrepreneur, esquire, fat cat, father,
   figure, financier, gentleman, grand duke, grandee, great man,
   hidalgo, important person, industrialist, interests, king,
   lace-curtain, laird, landgrave, lion, little businessman, lord,
   lordling, lords of creation, magnifico, man of commerce,
   man of mark, manager, margrave, marquis, merchant prince, mogul,
   nabob, name, noble, nobleman, notability, notable, optimate,
   palsgrave, panjandrum, patrician, peer, person of renown,
   personage, personality, pillar of society, plutocrat, power,
   power elite, prince, ruling circle, sachem, seigneur, seignior,
   silk-stocking, somebody, something, squire, swell, the great,
   the top, thoroughbred, top brass, top executive, top people,
   tycoon, upper-cruster, very important person, viscount, waldgrave,
   worthy