"Safety factor." It's probably one of the most common, and most misused, terms in engineering.
The "factor of safety" might be better described as a "factor of ignorance". It's a multiplier, applied to loads and/or structural components in a design, to account for things that weren't explicitly taken into account in the design calculations. Those things could include variations in material quality or workmanship, uncertain overload conditions, accidents, and other hard-to-predict conditions that could cause a failure.
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