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):

heighten
    v 1: become more extreme; "The tension heightened" [syn:
         heighten, rise]
    2: make more extreme; raise in quantity, degree, or intensity;
       "heightened interest"
    3: increase; "This will enhance your enjoyment"; "heighten the
       tension" [syn: enhance, heighten, raise]
    4: increase the height of; "The athletes kept jumping over the
       steadily heightened bars"
    5: make (one's senses) more acute; "This drug will sharpen your
       vision" [syn: sharpen, heighten]
    6: make more intense, stronger, or more marked; "The efforts
       were intensified", "Her rudeness intensified his dislike for
       her"; "Pot smokers claim it heightens their awareness"; "This
       event only deepened my convictions" [syn: intensify,
       compound, heighten, deepen]

Moby Thesaurus II by Grady Ward, 1.0:

100 Moby Thesaurus words for "heighten":
   accelerate, add to, agent provocateur, aggrandize, aggravate,
   amplify, annoy, augment, beef up, better, blow up, boost, build,
   build up, buoy up, cast up, complicate, compound, concentrate,
   condense, consolidate, deepen, deteriorate, double, elevate,
   embitter, enhance, enlarge, erect, escalate, exacerbate,
   exaggerate, exasperate, expand, extend, heat up, heave, heft,
   heist, hike, hoick, hoist, hold up, hop up, hot up, improve,
   increase, intensify, irritate, jazz up, jerk up, key up, knock up,
   levitate, lift, lift up, lob, loft, magnify, make acute,
   make complex, make worse, mount, multiply, perk up, provoke, raise,
   raise up, ramify, rear, rear up, redouble, reinforce, rise, rouse,
   set up, sharpen, sky, soup up, sour, step up, stick up, strengthen,
   supplement, throw up, triple, up, upbuoy, upcast, upheave, uphoist,
   uphold, uplift, upraise, uprear, upsurge, upthrow, wax, whet,
   worsen