The Collaborative International Dictionary of English v.0.48:

Thane \Thane\ (th[=a]n), n. [OE. thein, [thorn]ein, AS.
   [thorn]egen, [thorn]egn; akin to OHG. degan a follower,
   warrior, boy, MHG. degen a hero, G. degen hero, soldier,
   Icel. [thorn]egn a thane, a freeman; probably akin to Gr.
   te`knon a child, ti`ktein to bear, beget, or perhaps to Goth.
   [thorn]ius servant, AS. [thorn]e['o]w, G. dienen to serve.]
   A dignitary under the Anglo-Saxons and Danes in England. Of
   these there were two orders, the king's thanes, who attended
   the kings in their courts and held lands immediately of them,
   and the ordinary thanes, who were lords of manors and who had
   particular jurisdiction within their limits. After the
   Conquest, this title was disused, and baron took its place.
   [1913 Webster]

   Note: Among the ancient Scots, thane was a title of honor,
         which seems gradually to have declined in its
         significance. --Jamieson.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Baron \Bar"on\, n. [OE. baron, barun, OF. baron, accus. of ber,
   F. baron, prob. fr. OHG. baro (not found) bearer, akin to E.
   bear to support; cf. O. Frisian bere, LL. baro, It. barone,
   Sp. varon. From the meaning bearer (of burdens) seem to have
   come the senses strong man, man (in distinction from woman),
   which is the oldest meaning in French, and lastly, nobleman.
   Cf. L. baro, simpleton. See Bear to support.]
   [1913 Webster]
   1. A title or degree of nobility; originally, the possessor
      of a fief, who had feudal tenants under him; in modern
      times, in France and Germany, a nobleman next in rank
      below a count; in England, a nobleman of the lowest grade
      in the House of Lords, being next below a viscount.
      [1913 Webster]

   Note: "The tenants in chief from the Crown, who held lands of
         the annual value of four hundred pounds, were styled
         Barons; and it is to them, and not to the members of
         the lowest grade of the nobility (to whom the title at
         the present time belongs), that reference is made when
         we read of the Barons of the early days of England's
         history. . . . Barons are addressed as `My Lord,' and
         are styled `Right Honorable.' All their sons and
         daughters are `Honorable.'" --Cussans.
         [1913 Webster]

   2. (Old Law) A husband; as, baron and feme, husband and wife.
      [R.] --Cowell.
      [1913 Webster]

   Baron of beef, two sirloins not cut asunder at the
      backbone.

   Barons of the Cinque Ports, formerly members of the House
      of Commons, elected by the seven Cinque Ports, two for
      each port.

   Barons of the exchequer, the judges of the Court of
      Exchequer, one of the three ancient courts of England, now
      abolished.
      [1913 Webster]

WordNet (r) 3.0 (2006):

baron
    n 1: a nobleman (in various countries) of varying rank
    2: a British peer of the lowest rank
    3: 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:

116 Moby Thesaurus words for "baron":
   Brahman, Establishment, VIP, archduke, aristocracy, aristocrat,
   armiger, banker, baronet, barons, big boss, big businessman,
   big gun, big man, big name, bigwig, blue blood, brass, brass hat,
   business leader, businessman, captain of industry, celebrity,
   count, cream, czar, daimio, dignitary, dignity, director, duke,
   earl, elder, elite, enterpriser, entrepreneur, esquire,
   establishment, 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, magnate,
   magnifico, man of commerce, man of mark, manager, margrave,
   marquis, merchant prince, mogul, nabob, name, nobility, noble,
   nobleman, notability, notable, optimate, overlapping, palsgrave,
   panjandrum, patrician, peer, person of renown, personage,
   personality, pillar of society, power, power elite,
   power structure, prince, ruling circle, ruling circles,
   ruling class, sachem, seigneur, seignior, silk-stocking, somebody,
   something, squire, swell, the best, the best people, the brass,
   the great, the top, thoroughbred, top brass, top executive,
   top people, tycoon, upper class, upper crust, upper-cruster,
   very important person, viscount, waldgrave, worthy



Bouvier's Law Dictionary, Revised 6th Ed (1856):

BARON. This word has but one signification in American law, namely, husband:
we use baron and feme, for husband and wife. And in this sense it is going
out of use.
     2. In England, and perhaps some other countries, baron is a title of
honor; it is the first degree of nobility below a viscount. Vide Com. Dig.
Baron and Feme; Bac. Ab. Baron and Feme; and the articles. Husband;
Marriage; Wife.
     3. In the laws of the middle ages, baron or bers, (baro) signifies a
great vassal; lord of a fief and tenant immediately from the king: and the
words baronage, barnage and berner, signify collectively the vassals
composing the court of the king; as Le roi et son barnage, The king and his
court. See Spelman's  Glossary, verb. Baro.