Source : Webster's Revised Unabridged Dictionary (1913)

Impossible \Im*pos"si*ble\, n.
   An impossibility. [Obs.]

         ``Madam,'' quoth he, ``this were an impossible!''
                                                  --Chaucer.

Impossible \Im*pos"si*ble\, a. [F., fr. L. impossibilis; pref.
   im- not + possibilis possible. See {Possible}.]
   Not possible; incapable of being done, of existing, etc.;
   unattainable in the nature of things, or by means at command;
   insuperably difficult under the circumstances; absurd or
   impracticable; not feasible.

         With men this is impossible; but with God all things
         are possible.                            --Matt. xix.
                                                  26.

         Without faith it is impossible to please him. --Heb.
                                                  xi. 6.

   {Impossible quantity} (Math.), an imagnary quantity. See
      {Imaginary}.

   Syn: See {Impracticable}.

Source : WordNet®

impossible
     adj 1: not capable of occurring or being accomplished or dealt
            with; "an impossible dream"; "an impossible situation"
            [ant: {possible}]
     2: totally unlikely [syn: {inconceivable}, {out of the question},
         {unimaginable}]
     3: used of persons or their behavior; "impossible behavior";
        "insufferable insolence" [syn: {insufferable}, {unacceptable},
         {unsufferable}]

impossible
     n : something that cannot be done; "his assignment verged on the
         impossible"