Tautology definitions

tô-tŏl'ə-jē
An empty or vacuous statement composed of simpler statements in a fashion that makes it logically true whether the simpler statements are factually true or false; for example, the statement Either it will rain tomorrow or it will not rain tomorrow.
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A proposition that is analytic.
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The definition of a tautology is a statement that says the same thing twice in different ways, or a statement that has to be true by the way it is phrased.

The statement "the reason is because" is an example of a tautology because "the reason" and "because" are essentially the same thing said twice in different ways.

The statement "it is either right or it is left" is an example of a tautology because the statement by definition always has to be true.

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Needless repetition of the same sense in different words; redundancy.
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An instance of such repetition.
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Needless repetition of an idea in different words; redundancy; pleonasm (Ex.: “necessary essentials”)
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An instance of such repetition.
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(uncountable) Redundant use of words.

It is tautology to say, "Forward Planning".

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(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".

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(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.

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Origin of tautology

From Late Latin tautologia, from Ancient Greek ταυτολογία (tautología) from ταὐτός (tautós, “the same") + λόγος (lógos, “explanation")