An element or system that is intended to be used only in the event that the primary element or system is non-functional.
Source: API SPEC 16D, Specification for Control Systems for Drilling Well Control Equipment and Control Systems for Diverter Equipment, Upstream Segment, Second Edition, July 2004. Global Standards
Redundant equipment available to complete an operation in the event the primary equipment fails.
Source: IADC UBO / MPD Glossary, December 2011. Global Standards