|This article/section is a stub — probably a pile of half-sorted notes and is probably a first version, is not well-checked, so may have incorrect bits. (Feel free to ignore, or tell me)
Pleonasm refers to overabundance. Not just in language, but that is our focus here.
Around language, pleonasm usually refers to phrases with more words than necessary, often by being redundant, repetitive, having empty words, cliche - but not necessarily wrong or confusing.
It can also refer to unremarkable use of idiom - doesn't add anything. Pleonasm can serve as a redundancy check; if a word is unknown, misunderstood, misheard, or if the medium of communication is poor—a static-filled radio transmission or sloppy handwriting—pleonastic phrases can help ensure that the meaning is communicated even if some of the words are lost.[citation needed
There is then a (fuzzy) division into:
- semantic pleonasm, where words mean the same things once you take their sense. For example, "Receive a free gift"
- perfectly omissible words - because of semantics or syntactics
- where the meaning would be the same without some words (semantics)
- and possibly even the way you parse the sentence (syntax)
- A number of idioms and idiom-like jargon ("null and void", "terms and conditions") can be fairly pleonastic.
- Some redundancies in abbreviations (PIN number), loan words, and sentences that use them can be considered subtle pleonasms.
- You can often insert prepositions with no change in meaning
- In informal language, you can often drop pronouns (and other pro-forms, although this arguably has more to do with laziness than pleonasm
- in many sentences you can use or omit (copula-style) 'that' is often not strictly necessary -- though it often does guide readers to the correct reading (avoids some garden paths)
Pleonasm can be stylistic in that it can be a means of drawing attention (to intent, a point, detail, or such). You could include
- double negatives
- wordiness in poetic description
- exact description, for example in legal texts
- Circumlocution (similar, intentional confusion)
A rhetorical tautology refers to a repetition of the same meaning (sometimes out of verbose style, sometimes simply unintentional).
(A logical tautology is something true under any possible case or interpretation. In formal/propositional logic, a boolean expression that is true regardless of values is a tautology.)
Many things called tautologies lie somewhere inbetween.
For example, when style guides recommend "Do not explain too much", that's a pretty tautologous, circular-logic suggestion: too much is by definition too much, and the suggestion should have some measure of how much is too much.
Difference between pleonasm and tautology
The two concepts overlap in the sense of needless verbosity/repetition.
Pleonasm has a sense of using an unnecessary overabundance of redundant words in one description.
Tautology has a sense of saying the exact same in different words, using multiple words with the same meaning.
You could argue that pleonasm is a specific manifestation of tautology.