An Etymological Dictionary of Astronomy and Astrophysics

1. Needless repetition of an idea, especially in words other than those of the immediate context, without imparting additional force or clearness, as in “lifeless dead.”
   

2. A → proposition that is → always  → true. On a → truth table a tautology is a → sentence that is true for every possible → truth value of its constituent parts.

Etymology (EN): L.L. tautologia “representation of the same thing in other words,” from Gk. tautologia, from tautologos “repeating what has been said,” from tauto “the same” (contraction of to auto “the same,” from to “the” + → auto + -logos “saying,” related to legein “to say,” → -logy.

Etymology (PE): Hamân “same” (Mid.Pers. ham “same; also; together,” → com-) + ân “that.”