Reliability

1. Reliability; Origin, Meaning, and Scope: Processes, components, equipment, systems, and people are not perfect and not free from failures. In a simplistic and deterministic view, we can have perfection with perfect reliability. In the real world we fall short of perfection (perfection exists only in a fantasy world). Everything fails either because of events or from aging deteriorations. A natural law of entropy express   the lowest energy state as a failure buildings always fall down, they never fall up which means we must continually maintain processes and equipment to prevent disorder   and failures. This requires spending time and resources to mitigating failure effects as nothing lasts forever.
Reliability is the probability that a component, system, or process will function without failure for a specified length of time when operated correctly under specified conditions. Reliability engineering is a strategic task concerned with predicting and avoiding failures.   For quantifying reliability issues it is important to know why, how, how often and cost of failures. Reliability issues are bound to the physics of failure mechanism so the failure mechanism can be mitigated. In the real world all potential failures are seldom well known which makes failure prediction a probabilistic issue for reliability analysis.
Reliability is not the same as availability although both are described as a value between 0 and 1. Availability tells the percent of time the system is alive and ready for use if called upon, and system factors define the actual online times as a percentage of up time. Reliability addresses the probability for a failure free interval under specific conditions. Reliability is the sweet absence of failure. Uncertainty is the sour presence of failures which cost money.  
The first major committee on reliability was set up by the US Department of defence in 1950. This was later called the Advisory Group on Reliability of Electronics Equipment (AGREE)....