tautology

Etymology

From Late Latin tautologia, from Ancient Greek ταὐτολογία (tautología) from ταὐτός (tautós, “the same”) + λόγος (lógos, “explanation”), analyzed as tauto- + -logy.

noun

1. (uncountable) Redundant use of words, a pleonasm, an unnecessary and tedious repetition.
   It is tautology to say, "Forward Planning".
2. (countable) An expression that features tautology.
   The expression "raze to the ground" is a tautology, since the word "raze" includes the notion "to the ground".
   Pure mathematics consists of tautologies, analogous to ‘men are men’, but usually more complicated. 1946, Bertrand Russell, History of Western Philosophy
3. (countable, logic, propositional logic) A statement that is true for all truth values of its propositional variables.
4. (countable, logic, first-order logic) A statement that is true for all truth values of its Boolean atoms.