Definition of "tautology"

(uncountable) Redundant use of words, a pleonasm, an unnecessary and tedious repetition.  examples

(countable) An expression that features tautology.  quotations  examples

(countable, logic, propositional logic) A statement that is true for all truth values of its propositional variables.  examples

(countable, logic, first-order logic) A statement that is true for all truth values of its Boolean atoms.  examples