The Collaborative International Dictionary of English v.0.48:

Heighten \Height"en\ (h[imac]t"'n), v. t. [Written also
   highten.] [imp. & p. p. Heightened; p. pr. & vb. n.
   Heightening.]
   1. To make high; to raise higher; to elevate.
      [1913 Webster]

   2. To carry forward; to advance; to increase; to augment; to
      aggravate; to intensify; to render more conspicuous; --
      used of things, good or bad; as, to heighten beauty; to
      heighten a flavor or a tint. "To heighten our confusion."
      --Addison.
      [1913 Webster]

            An aspect of mystery which was easily heightened to
            the miraculous.                       --Hawthorne.
      [1913 Webster]

WordNet (r) 3.0 (2006):

heightening
    adj 1: reaching a higher intensity; "their heightening fears"

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "heightening":
   accelerando, acceleration, aggrandizement, aggravation,
   amplification, annoyance, augmentation, ballyhoo, beefing-up,
   big talk, blowing up, blowup, burlesque, caricature, concentration,
   condensation, consolidation, contentiousness, deepening,
   deliberate aggravation, deterioration, dilatation, dilation,
   embittering, embitterment, enhancement, enlargement, exacerbation,
   exaggerating, exaggeration, exasperation, excess, exorbitance,
   expansion, explosion, extravagance, extreme, grandiloquence,
   heating-up, huckstering, hyperbole, hyperbolism, increase,
   inflation, information explosion, inordinacy, intensification,
   irritation, magnification, overemphasis, overestimation, overkill,
   overstatement, pickup, population explosion, prodigality,
   profuseness, provocation, puffery, puffing up, redoubling,
   reinforcement, sensationalism, sharpening, souring, speedup,
   step-up, strengthening, stretching, superlative, tall talk,
   tightening, touting, travesty, worsening