redundancy

Từ điển American Heritage Dictionary 4th

re·dun·dan·cy (rĭ-dŭnʹdən-sē)n. pl. re·dun·dan·cies
1. The state of being redundant.
2. Something redundant or excessive; a superfluity.
3. Repetition of linguistic information inherent in the structure of a language, as singularity in the sentence It works.
4. Excessive wordiness or repetition in expression.
5. Chiefly British.
a. The state or fact of being unemployed because work is no longer offered or considered necessary.
b. A dismissal of an employee from work for being no longer necessary; a layoff.
6. Electronics. Duplication or repetition of elements in electronic equipment to provide alternative functional channels in case of failure.
7. Repetition of parts or all of a message to circumvent transmission errors. 
Usage Note: The usages that critics have condemned as redundancies fall into several classes. Some expressions, such as old adage, mental telepathy, and VAT tax have become fixed expressions and seem harmless enough. In some cases, such as consensus of opinion, hollow tube, and refer back, the use of what is regarded as an unnecessary modifier or qualifier can sometimes be justified on the grounds that it in fact makes a semantic contribution. Thus a hollow tube can be distinguished from one that has been blocked up with deposits, and a consensus of opinion can be distinguished from a consensus of judgments or practice. Some locutions, such as close proximity, have been so well established that criticizing them may seem petty. See Usage Note at rarely. See Usage Note at refer. 
Our Living Language Although certain vernacular constructions, such as the double comparative and superlative constructions (as in more higher and most fastest) are scorned as unschooled redundancies, many fundamental features of Standard English, such as subject-verb agreement, also manifest redundancy in their double marking. For example, in She sits on the chair, the -s inflection on sit indicates that the subject of the sentence is a third-person-singular form. However, this information is redundant—it is conveyed by she. Standard English pronoun forms may encode redundancies too. Subject pronouns are nominative, and direct object pronouns are objective (for example, I saw him and He saw me); these distinctive forms are technically not necessary, however, since normal English word order makes evident whether a pronoun refers to a subject or object. Nevertheless, standard practice requires the avoidance of constructions such as double comparatives except when they are employed for rhetorical or comical effect.