Redundancy is an important means of achieving reliability if one component fails, there are alternative ways to achieve the function in question and the system performance is unaltered.
Having more engines than needed for fiight is an example of system redundancy.
Sending a piece of information in two independent ways through a system is another.
Segregation is a related concept.
Two parts of a system may both fail if they are physically (or temporally) too close; there may for example be a common cause of their failure due to their proximity, or the failure of one part may cause the failure of the other; for instance, if overload of an engine causes a fire that makes another engine fail.
Yet another related concept is diversity; redundant parts should have different realizations of the same function in order to avoid common cause failures, i.e., failures resulting from one common cause.
Redundant software systems, for example, should not be based on the same algorithms.
(A parallel procedural safeguard is here the classical rule for aviation pilots not to eat at the same restaurant before fiight in order to avoid
food poison.)