The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

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"

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "tautology":
   abundance, amplitude, battology, bedizenment, circumambages,
   circumbendibus, circumlocution, cloud of words, copiousness,
   diffuseness, diffusion, diffusiveness, duplication,
   duplication of effort, effusion, effusiveness, embellishment,
   expletive, extravagance, exuberance, fat, featherbedding,
   fecundity, fertility, filling, fluency, formlessness, frill,
   frills, frippery, gingerbread, gush, gushing, iteration, logorrhea,
   long-windedness, luxury, macrology, needlessness, ornamentation,
   outpour, overadornment, overflow, overlap, padding, palilogy,
   payroll padding, periphrase, periphrasis, pleonasm, prodigality,
   productivity, profuseness, profusion, prolificacy, prolificity,
   prolixity, rampancy, rankness, redundance, redundancy, reiteration,
   reiterativeness, repetition, repetition for effect,
   repetitiousness, repetitiveness, roundabout, stammering,
   stuttering, superabundance, superfluity, superfluousness,
   superflux, talkativeness, tautologism, teemingness, tirade,
   unnecessariness, verbality, verbiage, verbosity, wordiness



The Free On-line Dictionary of Computing (30 December 2018):

tautology

    A proposition which is always true.

   Compare: paradox.

   The Linguistic Smarandache Tautologies,
   (http://gallup.unm.edu/~smarandache/tautolog.txt).

   (1999-07-28)