Dependability is defined as "the trustworthiness of a component such that reliance can be justifiably placed on the service (the behavior as perceived by a user) it delivers." The reliability of a component is a measure of its continuous correct service delivery. A failure occurs when the service provided by the system no longer complies with its specification. An error is "that part of a system state which is liable to lead to failure" and a fault is defined as "the cause of an error".