The Collaborative International Dictionary of English v.0.48:

Enhance \En*hance"\, v. i.
   To be raised up; to grow larger; as, a debt enhances rapidly
   by compound interest.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Enhance \En*hance"\, v. t. [imp. & p. p. Enhanced; p. pr. &
   vb. n. Enhancing.] [Norm. F. enhauncer, enhaucer, OF.
   enhaleier, enhaucier; pref. en- (L. in) + haucier to lift,
   raise up, from an assumed L. altiare, fr. L. altus high; cf.
   Pr. enansar, enanzar, to advance, exalt, and E. advance. See
   Altitude, and cf. Hawser.]
   1. To raise or lift up; to exalt. [Obs.] --Wyclif.
      [1913 Webster]

            Who, naught aghast, his mighty hand enhanced.
                                                  --Spenser.
      [1913 Webster]

   2. To advance; to augment; to increase; to heighten; to make
      more costly or attractive; as, to enhance the price of
      commodities; to enhance beauty or kindness; hence, also,
      to render more heinous; to aggravate; as, to enhance
      crime.
      [1913 Webster]

            The reputation of ferocity enhanced the value of
            their services, in making them feared as well as
            hated.                                --Southey.
      [1913 Webster]

WordNet (r) 3.0 (2006):

enhance
    v 1: increase; "This will enhance your enjoyment"; "heighten the
         tension" [syn: enhance, heighten, raise]
    2: make better or more attractive; "This sauce will enhance the
       flavor of the meat"

Moby Thesaurus II by Grady Ward, 1.0:

97 Moby Thesaurus words for "enhance":
   accelerate, acculturate, add to, adorn, advance, agent provocateur,
   aggravate, ameliorate, amend, amplify, annoy, augment, beautify,
   become, beef up, better, blow up, boost, bring forward, build up,
   civilize, complement, complicate, concentrate, condense,
   consolidate, deepen, deteriorate, double, edify, educate, elevate,
   embellish, embitter, embroider, emend, enlarge, enlarge upon,
   enlighten, enrich, exacerbate, exaggerate, exalt, exasperate,
   fatten, favor, forward, foster, go straight, heat up, heighten,
   hop up, hot up, improve, improve upon, increase, intensify,
   irritate, jazz up, key up, lard, lift, magnify, make acute,
   make an improvement, make complex, make worse, meliorate, mend,
   mount, nurture, promote, provoke, raise, ramify, redouble,
   refine upon, reform, reinforce, rise, rouse, sharpen, socialize,
   soup up, sour, step up, straighten out, strengthen, suit, swell,
   transfigure, transform, triple, upgrade, uplift, whet, worsen