Redundancy, in general terms, refers to the quality or state of being redundant, that is: exceeding what is necessary or normal, containing an excess. This can have a negative connotation, superfluous, but also positive, serving as a duplicate for preventing failure of an entire system.

Computer

Sometimes in databases redundant data is stored, e.g. if in a personnel information database both the postal code and the full address of a person is stored, the street and city information is redundant as it is already enclosed in the postal code. See also Database normalization.

Engineering

The duplication of critical components of a system with the intention of increasing reliability of the system is called redundancy. In safety-critical systems, such as fly-by-wire aircraft, some parts of the control system may be triplicated. An error in one component then may then be out-voted by the other two. In a triply redundant system, the system has three sub components, all three of which must fail before the system fails. Since each one rarely fails, and the sub components are expected to fail independently, the probability of all three failing is calculated to be extremely small. See safety engineering.

Information theory

The number of bits used to transmit a message minus the number of bits of actual information in the message. Data compression is a way to eliminate such redundancy.

Language

In language, redundancy often takes the form of phrases which repeat a concept with a different word. Common examples in American English: added bonus, and plus, end result, free gift, future plans, hot water heater, new beginning, safe haven, past history, plan ahead, preplan, time period, tuna fish. (For other forms see Pleonasm.)

Law

In UK employment law, redundancy is the dismissal of an employee when his or her job becomes unnecessary. UK redundancy law allows three reasons for redundancy:

1. Total cessation of the employer's business (whether permanently or temporarily)
2. Cessation of business at the employee's workplace
3. Reduction in the number of workers required to do a particular job.

The law requires the employer to make a statutory redundancy payment, which is tax-free and is based on the employee's length of service, as long as the employee has served a minimum of two years. The employee is not allowed to claim redundancy if he or she was offered an alternative position with similar salary, status and responsibilities.

Total Quality Management

In total quality management, TQM, redundancy in quality or redundant quality means quality which exceeds the required quality level. Tolerances may be too accurate, for example, creating unnecessarily high costs of production.

Redundant quality is sometimes incorrectly used instead of even quality or constant quality, perhaps because of the positive connotations of the term redundancy used in connection with safety-critical systems.