Definition from Wiktionary, the free dictionary
- (uncountable) redundant use of words
- It is tautology to say, "Forward Planning".
- (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".
- 1946, Bertrand Russell, History of Western Philosophy:
- Pure mathematics consists of tautologies, analogous to ‘men are men’, but usually more complicated.
- (countable, logic) A statement that is true for all values of its variables
- Given a Boolean A, "A OR (NOT A)" is a tautology.
- A logical statement which is neither a tautology nor a contradiction is a contingency.
- A tautology can be verified by constructing a truth tree for its negation: if all of the leaf nodes of such truth tree end in X's, then the original (pre-negated) formula is a tautology.
Coordinate terms 
Derived terms 
terms derived from tautology (noun)
uncountable: redundant use of words
expression that features tautology