Reliability is the probability of success; where success is defined as compliance to specified requirements. A system or an item is considered reliable if it performs satisfactorily. Thus, reliability is an important attribute for estimating continuous operation of a system.
The main benefits by building reliable systems are time & cost. Usually, wherever a failure occurs, faulty part can always be replaced by a new part or it can be repaired. However, the materials manpower and the system down time are critical factors, which ultimately result in increased cost. The time required for repair translates into non-operational delays and for commercial organizations, this also means poor reputation; which in turn also affects growth of the organization. Thus, lack of reliability or poor reliability leads to a failure, which in turn results in user inconvenience, and financial loss.
Reliability of a system (or a product) is important.Manufacturers of consumer items claim reliability of their products in different terminology. e.g. a refrigeration company providing guarantee for 'X' years, but lighting manufacturers claim 'Y' hours of operation or a TV manufacturing company offers 'Z' years of warranty. When a manufacturer provides a guarantee / warranty for a definite duration, it is ensured and demonstrated by exhaustive testing carried out to establish trouble free operation.
Other terms related to reliability are "availability" and "survivability". Availability and maintainability apply to repairable systems only. Availability is a measure of degree to which the system can perform the required function. Availability is defined as a function of operating time and down time. (Down time includes repair time). Maintainability is a measure of to what extent a system can be retained in the desired condition and level of performance after repairs.
For an Earth station of Aircraft, the degree of maintainability plays an important role in its long term availability. For non-repairable systems like Satellite Launch Vehicle, missile, missile launcher etc., the measure of reliability is survivability. That means the system is required to perform, under specified operating conditions for a specified duration of time without failure.Hence it is notable that reliability is a quantitative (probabilistic) factor, which can be predicted in design, measurable in tests, assurable in production (and maintainable in field for repairable systems).