The Collaborative International Dictionary of English v.0.48:

Considerable \Con*sid"er*a*ble\ (k[o^]n*s[i^]d"[~e]r*[.a]*b'l),
   a. [Cf. F. consid['e]rable.]
   1. Worthy of consideration; requiring to be observed, borne
      in mind, or attended to.
      [1913 Webster]

            It is considerable, that some urns have had
            inscriptions on them expressing that the lamps were
            burning.                              --Bp. Wilkins.
      [1913 Webster]

            Eternity is infinitely the most considerable
            duration.                             --Tillotson.
      [1913 Webster]

   2. Of some distinction; noteworthy; influential; respectable;
      -- said of persons.
      [1913 Webster]

            You are, indeed, a very considerable man. --Junius.
      [1913 Webster]

   3. Of importance or value.
      [1913 Webster]

            In painting, not every action, nor every person, is
            considerable enough to enter into the cloth.
                                                  --Dryden.
      [1913 Webster]

            A considerable sum of money.          --Prescott.
      [1913 Webster]

WordNet (r) 3.0 (2006):

considerable
    adj 1: large or relatively large in number or amount or extent
           or degree; "a considerable quantity"; "the economy was a
           considerable issue in the campaign"; "went to
           considerable trouble for us"; "spent a considerable
           amount of time on the problem" [ant: inconsiderable]

Moby Thesaurus II by Grady Ward, 1.0:

128 Moby Thesaurus words for "considerable":
   able, absolute, active, appreciable, ascendant, authoritarian,
   authoritative, authorized, autocratic, big, big-league, big-name,
   big-time, bigwig, bigwigged, billion, bumper, capable,
   clothed with authority, commanding, competent, comprehensive,
   consequential, controlling, decent, deep, distinguished, dominant,
   double-barreled, duly constituted, earthshaking, effective,
   efficacious, eminent, empowered, estimable, ever so many,
   ex officio, exhaustive, extensive, fair, full, full many, good,
   goodly, governing, grand, grave, great, healthy, heaped-up, heavy,
   heavyweight, hefty, hegemonic, hegemonistic, high-powered,
   illustrious, imperative, important, influential, intense,
   irresistible, jillion, large, large-scale, leading, main, major,
   man-sized, many, material, maximum, meaningful, mighty, million,
   momentous, monocratic, multifarious, multifold, multiple,
   multitudinal, multitudinous, myriad, name, no few, not a few,
   notable, noteworthy, noticeable, numerous, of consequence,
   official, plenary, potent, powerful, preeminent, prestigious,
   pretty, prominent, puissant, quite some, ranking, remarkable,
   respectable, ruling, self-important, senior, sensible, serious,
   significant, sizable, sizeable, smart, strong, substantial,
   superior, supreme, tall, thousand, tidy, total, totalitarian,
   very many, weighty, world-shaking, worthy, zillion