What is the difference between these two words? They seem to mean the same thing.

  • 1
    Wikipedia says "Tautology and pleonasm are not consistently differentiated in literature."
    – Stuart F
    Commented Nov 7, 2022 at 16:55
  • My math teacher asks me to determine whether the equation 5 - 1 = 4 is true. I add 1 to each side of the equation, so on the next line I write 5 = 5. That line is a tautology (because it is clearly always true) but not a pleonasm (because it is necessary to complete the proof). Commented Nov 8, 2022 at 17:47

3 Answers 3


A pleonasm relates to a specific word or phrase where there is redundancy (a "true fact"), whereas a tautology relates more to a logical argument or assertion being made, where it is self-evidently true (or unable to be falsified by logic), such as "I was definitely the oldest person at the meeting because everyone there was born later than me.", or "For the purpose of paying fees, the year is divided into four quarterly payments.".

  • A good answer is expected to have supporting references, linked and attributed; answers lacking references come over as, and may merely be, opinion (with no guarantee of being correct or comprehensive). // I don't think there is a consensus that these are the only definitions in use. Commented May 13, 2020 at 18:37
  • I think the "tautology" mentioned here refers to tautology in the mathematical logic sense, and not in the linguistics sense (although the examples don't match the explanation given). In the linguistics sense, tautology means to repeat the same meaning twice using different words/phrases (e.g. "they arrived one after another in succession" - "one after another" and "in succession" have the same meaning but are phrased differently). Also, whether rightly or not, people use the two terms interchangeably.
    – d4nyll
    Commented Dec 18, 2021 at 0:52

A Pleonasm uses a descriptor or modifier to express a quality that is necessarily inherent to the word described or modified. Examples: Round circle, falling rain, foreign immigrants, two-wheeled bicycle, reflective mirror, stingy miser, brave heroes, clever genius, close proximity.

A Tautology is a repetition of synonyms to describe or modify a word. He quickly and speedily went away. A tiny little dog. A great big man. Tautology is often used for dramatic effect.

  • 1
    Some references would be helpful. Have a look at the Wikipedia pages, or try some other trustworthy reference work.
    – Stuart F
    Commented Nov 7, 2022 at 16:57

The two terms are closely related in meaning, but they are applied to different things. Only a complete sentence/proposition/statement can be said to be a tautology, while pleonasm can be applied to something that does not amount to a complete sentence/proposition/statement. For example,

This circle is round.

is a tautology, because it is true solely in virtue of the meanings of the words that it contains. On the other hand, the expression

round circle

is a pleonasm, because the meaning of the adjective round is already contained in the meaning of the noun, circle that it qualifies, but it is not a tautology, because, by itself, it does not amount to a complete sentence; in fact it would be meaningless to ask whether it is a tautology.

This is how tautology is used in the terminology of logic; as has already been observed in a comment, outside logic, the two terms are used more loosely, and often interchangeably.

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