Definition of tautology

1. Redundant use of words.
It is tautology to say, "Forward Planning".
2. An expression that features tautology.
The expression "raze to the ground" is a tautology, since the word "raze" includes the notion "to the ground".
3. 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.
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Example Sentences

Both follow from the equivalence theorem and the tautology theorem.
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That's the nature of tautologies.
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This is one reason why tautologies are important.
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I do not think I have ever come across such a fine exercise in tautology in a public document.
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We keep getting caught in a huge tautology and I am afraid that this is precisely the economic policy which you support.
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But a statement like that is in itself little more than tautology.
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And it's not ATMOS-system, that's a tautology.
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But, though this might appear a tautology, there is much that can be done by the United Nations to promote a culture of peace in actions taken to promote peace and security.
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Could it be that the Help file writer for this feature didn't know either, and made something up that represented a tautology?
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In draft guideline 3.1.1, the chapeau and the first line immediately following it were a tautology.
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