Definition of tautologies:
(n) :
(uncountable) Redundant use of words, a pleonasm, an unnecessary and tedious repetition.
(n) :
(countable, logic, propositional logic) A statement that is true for all truth values of its propositional variables.
(n) :
(countable, logic, first-order logic) A statement that is true for all truth values of its Boolean atoms.
(n) :
(countable) An expression that features tautology.