In the study of language, redundancy is considered a vital feature of language. It shields a message from possible flaws in transmission (unclarity, ambiguity, noise). In this way, it increases the odds of predictability of a message's meaning. On the phonological level, the redundancy of phonological rules may clarify some vagueness in spoken speech; "a speaker may know that 'thisrip' must be 'this rip' and not 'this srip' because the English consonant cluster 'sr' is illegal'" (Pinker, 1994, p. 178).
It is this feature of redundancy that has been said to be important in allowing humans to acquire a complex grammar system. A child acquiring language must abstract away grammatical rules based on the input which s/he hears. Redundancy in language allows the child's inductions to be more stable by presenting more salient evidence upon which these inductions are based. Redundancy therefore provides the sufficient stimulus needed to acquire a complex grammar system.
A common concept in linguistics is economy of storage; only unpredictable information is said to be stored in one's "mental grammar". The rest must be reconstructed by the speaker in conversation, or "on-line", as this has been called. Redundancy aids this process, increasing the odds of predictability by acting as a "noise" filter.
In rhetoric
In the use of language, redundancy is the use of duplicative, unnecessary or useless wording. Some also expand the definition to include self-contradictory wording.
It most often takes the form of tautology: phrases which repeat a concept with different words. Common examples in American English: "an added bonus", "and plus", "end result", "free gift", "future plans", "hot water heater", "unconfirmed rumor", "killed him dead", "past history", "safe haven".
A subset of tautology is "RAS syndrome": "ATM machine", "HIV virus", "RAID array" (these phases expand to "automated teller machine machine", "human immunodeficiency virus virus", and "redundant array of independent disks array", respectively).
A more general classification of redundancy is pleonasm, which can be any unnecessary words (or even word parts). Subsuming both rhetorical tautology and RAS syndrome, it also includes dialectal usage of technically unnecessary parts, as in "off of" vs. "off", "onto" vs. "on", "know that it happened" vs "know it happened", etc. Pleonasm can also take the form of purely semantic redundancies that are a part of the de facto standard usage in a language and "transparent" to the user (e.g., the French question "Qu'est-ce que c'est?" meaning "What's that?" or "What is it?", which translates very literally as "What is it that it is?") The term pleonasm is most often, however, employed as synonymous with tautology.
The use of obfuscating, tumid linguistic constructions in vocally or graphically expressed communications (as in that phase, which could be more simply expressed as "being longwinded") is also a form of redundancy, with several names. Two rather formal names for it are prolixity and logorrhoea. It is often done with manipulative intent, e.g. to confuse and mislead the audience, to disguise the actual nature of a position or fact, or persuade in politics or religion. In such cases it is often also fallacious. Comedian George Carlin is famous for criticizing the politically and socially motived abuse of logorrhea to hide the truth or manipulate public perception.
Finally, a borderline type of construction that could be considered redundancy (in that it is an extension of pleonasm) is the oxymoron, or self-contradictory expression, in which the unnecessary verbiage is not simply deadwood but undermines the meaning intended to be conveyed. A common example is "irregardless", a double negative that technically means the opposite of the intended real words "regardless" and "irrespective" that have become confused to yield "irregardless". Oxymora usually involve more than one word, however, as in "almost exactly", "centered around", and "genuine replica". Like prolixity, oxmora are often used to mislead or euphemize, though often they are simply the product of muddled logic and poor writing.
All of these forms of redundancy can be used intentionally, for positive artistic or rhetorical effect, frequently for humorous purpose, and for a number of other non-manipulative purposes, so their appearance in speech or writing is not automatically a fault.