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The provision of more than one means or parallel paths in a structure for performing a given function such that all means must fail before causing system failure.

The parallel structure in Figure 2.13 has redundancy because all the blocks have to fail to cause the specified system failure, SF. Because blocks have to fail, the system is said to have redundancy of order .

Parallel or redundant paths can be installed for a single block, for a selection of blocks, or for the entire system function, SF.

For hardware, redundancy may be achieved by installing one or more extra hardware items in parallel with the initial item. The redundant items may be identical or diverse. Adding redundancy increases the cost and makes the system more complicated, but if the cost of failure is high, redundancy is often an attractive option.