NOUN (2)

1. (logic) a statement that is necessarily true;
- Example: "the statement `he is brave or he is not brave' is a tautology";

2. useless repetition;
- Example: "to say that something is `adequate enough' is a tautology";

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"

The Free On-line Dictionary of Computing (19 January 2023):

tautology

    A proposition which is always true.

   Compare: paradox.

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

   (1999-07-28)