The Collaborative International Dictionary of English v.0.48:

Enlighten \En*light"en\, v. t. [Pref. en- + lighten: cf. AS.
   inl[imac]htan. Cf. Enlight.]
   1. To supply with light; to illuminate; as, the sun
      enlightens the earth.
      [1913 Webster]

            His lightnings enlightened the world. --Ps. xcvii.
                                                  4.
      [1913 Webster]

   2. To make clear to the intellect or conscience; to shed the
      light of truth and knowledge upon; to furnish with
      increase of knowledge; to instruct; as, to enlighten the
      mind or understanding.
      [1913 Webster]

            The conscience enlightened by the Word and Spirit of
            God.                                  --Trench.
      [1913 Webster]

WordNet (r) 3.0 (2006):

enlighten
    v 1: make understand; "Can you enlighten me--I don't understand
         this proposal" [syn: enlighten, edify]
    2: give spiritual insight to; in religion [syn: enlighten,
       irradiate]
    3: make free from confusion or ambiguity; make clear; "Could you
       clarify these remarks?"; "Clear up the question of who is at
       fault" [syn: clear, clear up, shed light on,
       crystallize, crystallise, crystalize, crystalise,
       straighten out, sort out, enlighten, illuminate,
       elucidate]

Moby Thesaurus II by Grady Ward, 1.0:

158 Moby Thesaurus words for "enlighten":
   account for, acculturate, acquaint, advance, advertise,
   advertise of, advise, allegorize, ameliorate, amend, apprise,
   awaken, beacon, better, boost, break the spell, brief, brighten,
   brighten up, bring forward, bring word, broaden the mind,
   burst the bubble, catechize, civilize, clarify, clear up, coach,
   communicate, correct, counsel, crack, debunk, decipher,
   demonstrate, demythologize, direct, disabuse, disappoint, disclose,
   disenchant, disillude, disillusion, disillusionize, edify, educate,
   elevate, elucidate, emend, enhance, enrich, euhemerize, exemplify,
   explain, explain away, explicate, expose, exposit, expound,
   familiarize, fatten, favor, flood with light, floodlight, forward,
   foster, give instruction, give lessons in, give notice,
   give reason for, give the facts, give the meaning, give word,
   go straight, ground, guide, highlight, illume, illuminate,
   illumine, illustrate, improve, improve upon, inform, instruct,
   irradiate, lard, leave word, let down easy, let in on, let know,
   lift, light, light up, lighten, luminate, make an improvement,
   make clear, make plain, meliorate, mend, mention to, notify,
   nurture, open the eyes, overshine, popularize, prick the bubble,
   promote, put straight, raise, rationalize, reeducate, refine upon,
   reform, relume, relumine, report, school, send word, serve notice,
   set right, set straight, sharpen the wits, shed light upon,
   shine upon, show, show how, show the way, show up, simplify,
   socialize, solve, speak, spell out, spotlight, straighten out,
   teach, teach a lesson, teach the rudiments, tell, tell the truth,
   throw light upon, train, transfigure, transform, tutor,
   unblindfold, uncharm, undeceive, unfold, unlock, unravel, unspell,
   upgrade, uplift, verse, wake up