The Collaborative International Dictionary of English v.0.48:

Borough \Bor"ough\, n. [See Borrow.] (O. Eng. Law)
      (a) An association of men who gave pledges or sureties to
          the king for the good behavior of each other.
      (b) The pledge or surety thus given. --Blackstone.
          Tomlins.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Borough \Bor"ough\, n. [OE. burgh, burw, boru, port, town,
   burrow, AS. burh, burg; akin to Icel., Sw., & Dan. borg, OS.
   & D. burg, OHG. puruc, purc, MHG. burc, G. burg, Goth.
   ba['u]rgs; and from the root of AS. beorgan to hide, save,
   defend, G. bergen; or perh. from that of AS. beorg hill,
   mountain. [root]95. See Bury, v. t., and cf. Burrow,
   Burg, Bury, n., Burgess, Iceberg, Borrow, Harbor,
   Hauberk.]
   1. In England, an incorporated town that is not a city; also,
      a town that sends members to parliament; in Scotland, a
      body corporate, consisting of the inhabitants of a certain
      district, erected by the sovereign, with a certain
      jurisdiction; in America, an incorporated town or village,
      as in Pennsylvania and Connecticut. --Burrill. --Erskine.
      [1913 Webster]

   2. The collective body of citizens or inhabitants of a
      borough; as, the borough voted to lay a tax.
      [1913 Webster]

   Close borough, or Pocket borough, a borough having the
      right of sending a member to Parliament, whose nomination
      is in the hands of a single person.

   Rotten borough, a name given to any borough which, at the
      time of the passage of the Reform Bill of 1832, contained
      but few voters, yet retained the privilege of sending a
      member to Parliament.
      [1913 Webster]

WordNet (r) 3.0 (2006):

borough
    n 1: one of the administrative divisions of a large city
    2: an English town that forms the constituency of a member of
       parliament

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "borough":
   Kreis, Stadt, archbishopric, archdiocese, arrondissement,
   bailiwick, banlieue, bishopric, boom town, bourg, burg, burgh,
   canton, city, close borough, commune, congressional district,
   constablewick, conurbation, county, departement, diocese, district,
   duchy, election district, electoral district, electorate, exurb,
   exurbia, faubourg, gerrymander, gerrymandered district, ghost town,
   government, greater city, hamlet, hundred, magistracy, market town,
   megalopolis, metropolis, metropolitan area, municipality, oblast,
   okrug, outskirts, parish, pocket borough, polis, precinct,
   principality, province, region, riding, rotten borough,
   safe district, sheriffalty, sheriffwick, shire, shrievalty,
   silk-stocking district, single-member district, soke, spread city,
   stake, state, suburb, suburbia, swing district, territory, town,
   township, urban complex, urban sprawl, urbs, village, ville,
   wapentake, ward



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

BOROUGH. An incorporated town; so called in the charter. It is less than a
city. 1 Mann. & Gran. 1; 39 E. C. L. R. 323.