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The ability of an item to perform as required in a stated operating context and for a stated period of time.

The term item is used to designate any technical system, subsystem, or component. The items studied in this book are built of hardware parts, and to an increasing degree, of software. When relevant, the user interface is part of the item, but operators and other humans are not part of the items studied here.

The reliability concept is illustrated in Figure 1.1. The required performance is determined by laws and regulations, standards, customer requirements and expectations, and supplier requirements, and is usually stated in a specification document, where delimitations of the operating context are stated. As long as the predicted performance at least fulfills the required performance, the item is reliable – when it is used in the same operating context and for the period of time stated in the required performance.

Figure 1.1 The reliability concept.

By operating context, we mean the environmental conditions the item is used in, the usage patterns, and the loads it is subjected to, and how the item is serviced and maintained.

Definition 1.1 is not new and is not created by us. Several authors and organizations have used this, or a very similar definition of reliability, at least since the 1980s. A more thorough discussion of reliability and related concepts is given in Section 1.3.