In literary criticism and rhetoric, a tautology is a statement which repeats the same idea, using near-synonymous morphemes, words, or phrases, that is, "saying the same thing twice". Tautology and pleonasm are not consistently differentiated in the literature.
Like pleonasm, it is often considered a fault of style when unintentional. On the other hand, an intentional repetition may be an effective way to emphasize a thought, or help the listener or reader understand a point.
The word was coined in Hellenistic Greek from ταὐτός ("the same") plus λόγος ("word/idea"), and transmitted through 3rd-century Latin tautologia and French tautologie. It first appears in English in the 16th century. The use of the term logical tautology was introduced in English by Wittgenstein in 1919, perhaps following Auguste Comte's usage in 1835.
Intentional repetition of meaning intends to amplify or emphasize a particular, usually significant, fact about what is being discussed. For example, a gift is, by definition, free of charge; using the phrase "free gift" might emphasize that there are no hidden conditions or fine print, be it the expectation of money or reciprocation, or that the gift is being given by volition.
This is related to the rhetorical device of hendiadys, where one concept is expressed through the use of two descriptive words or phrases. For example, "goblets and gold" meaning wealth, or "this day and age" meaning the present time. Superficially these expressions may seem tautological, but they are stylistically sound because the repeated meaning is just a way to emphasize the same idea.
The use of tautologies is, however, usually unintentional. For example, the phrases "mental telepathy", "planned conspiracies", and "small dwarfs" imply that there are such things as "physical telepathy, spontaneous conspiracies, and giant dwarfs."
Parallelism is not tautology, but rather a particular stylistic device. Much Old Testament poetry is based on parallelism: the same thing said twice, but in slightly different ways (Fowler puts it as pleonasm). However, modern biblical study emphasizes that there are subtle distinctions and developments between the two lines, such that they are usually not truly the "same thing." Parallelism can be found wherever there is poetry in the Bible: Psalms, the Books of the Prophets, and in other areas as well.
- Fowler, Henry Watson (1 April 1983), Gowers, Sir Ernest, ed., Modern English Usage (2nd ed.), Oxford University Press, ISBN 978-0-19-281389-3
- Bryson, Bill (29 July 1999), The Mother Tongue: The English Language, Penguin Books, ISBN 978-0-14-014305-8
- Bogdan Szymanek, "Remarks on Tautology in Word-Formation" in Laurie Bauer, et al., Semantics of Complex Words, Studies in Morphology 3, 2015, ISBN 3319141023, p. 142
- John Bascom, Philosophy of Rhetoric, p. 241
- Oxford English Dictionary, 3rd edition, 2014, s.v.
- Ernest Ament, "Glossary of Rhetorical Terms" 
- Kallan, Richard (2005). Armed Gunmen, True Facts, and Other Ridiculous Nonsense: A Compiled Compendium of Repetitive Redundancies. Pantheon Books. p. x. ISBN 978-0-375-42352-9.