An alteration (or maintenance) of the state of a system as mere happenstance of the design given a set of the circumstances in which it might be found.
An unintended function is never proactively driven by Development Requirements. Requirements can (however) be backfilled if the function is important enough once discovered (see Verification by Surprise). Backfill can make it look as if the miracle happened on purpose, so is much favored by the Marketing Department.
If requirements are not retroactively declared, unintended functions can be relied on only if explicitly addressed by a Selected Item Drawing (SID). The (selected) part will typically require 1) Qualification as a separate CI, or 2) Qualification as part of some superior CI. The SID approach works IFF the (newly)desired characteristics can be conclusively related to those that are unambiguously observable during the acceptance of every Serial Number (or Lot Number) of the original part.
Note that the owner of the original Intellectual Property cannot be held liable for the new functionality, because the new use is (by definition) outside their intent. Its a dicey proposition, at best, available only to the very smart or very stupid1.
Contrast with function.
Footnotes- It can be really, really difficult to tell the difference sometimes.[↩]