Pleonasm (/ˈpliː.əˌnæzəm/; from Ancient Greek πλεονασμός pleonasmós, from πλέον pléon 'to be in excess') is redundancy in linguistic expression, such as "black darkness", "burning fire", or "the man he said". It is a manifestation of tautology by traditional rhetorical criteria. Pleonasm may also be used for emphasis, or because the phrase has become established in a certain form. Tautology and pleonasm are not consistently differentiated in literature.

Most often, pleonasm is understood to mean a word or phrase which is useless, clichéd, or repetitive, but a pleonasm can also be simply an unremarkable use of idiom. It can aid in achieving a specific linguistic effect, be it social, poetic or literary. Pleonasm sometimes serves the same function as rhetorical repetition—it can be used to reinforce an idea, contention or question, rendering writing clearer and easier to understand. 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]}

Some pleonastic phrases are part of a language's idiom, such as tuna fish, chain mail and safe haven in American English. They are so common that their use is unremarkable for native speakers, although in many cases the redundancy can be dropped with no loss of meaning.

When expressing possibility, English speakers often use potentially pleonastic expressions such as It might be possible or perhaps it's possible, where both terms (verb might or adverb perhaps along with the adjective possible) have the same meaning under certain constructions. Many speakers of English use such expressions for possibility in general, such that most instances of such expressions by those speakers are in fact pleonastic. Others, however, use this expression only to indicate a distinction between ontological possibility and epistemic possibility, as in "Both the ontological possibility of X under current conditions and the ontological impossibility of X under current conditions are epistemically possible" (in logical terms, "I am not aware of any facts inconsistent with the truth of proposition X, but I am likewise not aware of any facts inconsistent with the truth of the negation of X"). The habitual use of the double construction to indicate possibility per se is far less widespread among speakers of most^{[citation needed]} other languages (except in Spanish; see examples); rather, almost all speakers of those languages use one term in a single expression:^{[dubious – discuss]}

In a satellite-framed language such as English, verb phrases containing particles that denote direction of motion are so frequent that even when such a particle is pleonastic, it seems natural to include it (e.g. "enter into").

Some pleonastic phrases, when used in professional or scholarly writing, may reflect a standardized usage that has evolved or a meaning familiar to specialists but not necessarily to those outside that discipline. Such examples as "null and void", "each and every", "cease and desist" are legal doublets that are part of legally operative language that is often drafted into legal documents. A classic example of such usage was that by the Lord Chancellor at the time (1864), Lord Westbury, in the English case of ex parte Gorely, when he described a phrase in an Act as "redundant and pleonastic". This type of usage may be favored in certain contexts. However, it may also be disfavored when used gratuitously to portray false erudition, obfuscate, or otherwise introduce verbiage, especially in disciplines where imprecision may introduce ambiguities (such as the natural sciences).

Examples from Baroque, Mannerist, and Victorian provide a counterpoint to Strunk's advocacy of concise writing:

There are various kinds of pleonasm, including bilingual tautological expressions, syntactic pleonasm, semantic pleonasm and morphological pleonasm: