The Collaborative International Dictionary of English v.0.48:

Conceal \Con*ceal"\ (k[o^]n*s[=e]l"), v. t. [imp. & p. p.
   Concealed (k[o^]n*s[=e]ld"); p. pr. & vb. n. Concealing.]
   [OF. conceler, L. concelare; con- + celareto hide; akin to
   AS. helan, G. hehlen, E. hele (to cover), helmet. See Hell,
   Helmet.]
   To hide or withdraw from observation; to cover; to cover or
   keep from sight; to prevent the discovery of; to withhold
   knowledge of.
   [1913 Webster]

         It is the glory of God to conceal a thing. --Prov. xxv.
                                                  2.
   [1913 Webster]

         Declare ye among the nations, . . . publish and conceal
         not.                                     --Jer. l. 2.
   [1913 Webster]

         He which finds him shall deserve our thanks, . . .
         He that conceals him, death.             --Shak.

   Syn: To hide; secrete; screen; cover; disguise; dissemble;
        mask; veil; cloak; screen.

   Usage: To Conceal, Hide, Disguise, Dissemble,
          Secrete. To hide is the generic term, which embraces
          all the rest. To conceal is simply not make known what
          we wish to keep secret. In the Bible hide often has
          the specific meaning of conceal. See --1 Sam. iii. 17,
          18. To disguise or dissemble is to conceal by assuming
          some false appearance. To secrete is to hide in some
          place of secrecy. A man may conceal facts, disguise
          his sentiments, dissemble his feelings, secrete stolen
          goods.
          [1913 Webster]

                Bur double griefs afflict concealing hearts.
                                                  --Spenser.
          [1913 Webster]

                Both dissemble deeply their affections. --Shak.
          [1913 Webster]

                We have in these words a primary sense, which
                reveals a future state, and a secondary sense,
                which hides and secretes it.      --Warburton.
          [1913 Webster]

WordNet (r) 3.0 (2006):

conceal
    v 1: prevent from being seen or discovered; "Muslim women hide
         their faces"; "hide the money" [syn: hide, conceal]
         [ant: show]
    2: hold back; keep from being perceived by others; "She conceals
       her anger well" [syn: conceal, hold back, hold in]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "conceal":
   becloud, befog, blanket, blind, bosom, bury, cache, camouflage,
   classify, cloak, cloud, cover, cover up, curtain, disguise,
   dissemble, distract attention from, ditch, eclipse, ensconce,
   enshroud, envelop, file and forget, gloss over, hide, hold out on,
   in petto, keep, keep back, keep between us, keep buttoned up,
   keep close, keep dark, keep from, keep hidden, keep in ignorance,
   keep mum, keep secret, keep snug, keep under cover,
   keep under wraps, make no sign, mask, never let on, not give away,
   not tell, obfuscate, obscure, occult, play dumb, screen, secrete,
   shade, shroud, slur over, stash, varnish, veil, whitewash,
   withhold