The Collaborative International Dictionary of English v.0.48:

Nabob \Na"bob\ (n[=a]"b[o^]b), n. [Hind. naw[=a]b, from Ar.
   naw[=a]b, pl. of n[=a]["i]b a vicegerent, governor. Cf
   Nawab.]
   1. A deputy or viceroy in India; a governor of a province of
      the ancient Mogul empire.
      [1913 Webster]

   2. One who returns to Europe from the East with immense
      riches: hence, any man of great wealth. "A bilious old
      nabob." --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nabob
    n 1: a governor in India during the Mogul empire [syn: nawab,
         nabob]
    2: a wealthy man (especially one who made his fortune in the
       Orient)

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "nabob":
   Daddy Warbucks, Establishment, VIP, baron, beg, beglerbeg, bey,
   big chief, big gun, big man, big name, bigwig, billionaire,
   bloated plutocrat, brass, brass hat, burgrave, capitalist,
   celebrity, collector, dey, dignitary, dignity, elder, eminence,
   eparch, exarch, fat cat, father, figure, gauleiter, governor,
   governor-general, great man, high-muck-a-muck, important person,
   interests, khedive, lieutenant governor, lion, lords of creation,
   magnate, man of mark, man of means, man of substance,
   man of wealth, millionaire, mogul, moneybags, moneyed man,
   multimillionaire, name, nawab, notability, notable, palatine,
   panjandrum, parvenu, person of renown, personage, personality,
   pillar of society, plutocrat, power, power elite, proconsul,
   provincial, rich man, richling, ruling circle, sachem, satrap,
   somebody, something, stadtholder, subahdar, tetrarch, the great,
   the top, top brass, top people, tycoon, vali,
   very important person, vice-king, viceroy, wali, warm man,
   wealthy man, worthy