Source : Webster's Revised Unabridged Dictionary (1913)

Tautology \Tau*tol"o*gy\, n. [L. tautologia, Gr. ?: cf. F.
   tautologie.] (Rhet.)
   A repetition of the same meaning in different words; needless
   repetition of an idea in different words or phrases; a
   representation of anything as the cause, condition, or
   consequence of itself, as in the following lines:

         The dawn is overcast, the morning lowers, And heavily
         in clouds brings on the day.             --Addison.

   Syn: Repetition.

   Usage: {Tautology}, {Repetition}. There may be frequent
          repetitions (as in legal instruments) which are
          warranted either by necessity or convenience; but
          tautology is always a fault, being a sameness of
          expression which adds nothing to the sense or the
          sound.

Source : WordNet®

tautology
     n 1: (logic) a statement that is necessarily true; "the statement
          `he is brave or he is not brave' is a tautology"
     2: useless repetition; "to say that something is `adequate
        enough' is a tautology"

Source : Free On-Line Dictionary of Computing

tautology
     
         A {proposition} which is always true.
     
        Compare: {paradox}.
     
        {The Linguistic Smarandache Tautologies,
        (http://www.gallup.unm.edu/~smarandache/tautolog.txt)}.
     
        (1999-07-28)