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 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". Redundancy aids this process, increasing the odds of predictability by acting as a "noise" filter.
In language, redundancy is the use of duplicative, unnecessary or useless wording. Some people expand the definition to include self-contradictory wording.
Redundancy typically takes the form of tautology: phrases that repeat a meaning with different words. Common examples are: "a variety of different", "an added bonus", "to over-exaggerate", "and plus", "and etc.", "end result", "free gift", "future plans", "hot water heater", "unconfirmed rumor", "killed him dead", "past history", "safe haven", "potential hazard", "completely surrounded", "false pretense". There is also the self-referential joke "organization" called "The Redundancy Society of Redundancy", also rendered as "Society of Redundancy Society".
A subset of tautology is RAS syndrome: "ATM machine", "HIV virus", "Personal Identification PIN number" and "RAID array". These phases expand to "automated teller machine machine", "human immunodeficiency virus virus", "personal identification personal identification number number", and "redundant array of independent disks array", respectively. "RAS syndrome" is itself a tongue-in-cheek example of the RAS syndrome in action; it expands to "Redundant Acronym Syndrome syndrome".
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 phrase, which could more simply be 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 to persuade in politics or religion. In such cases it is often also fallacious. Comedian George Carlin was famous for criticizing the politically—and socially—motivated abuse of logorrhea to hide the truth or manipulate public perception.
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. For example, duplicative language used as parallelism can have a strong rhetorical effect.