WordNet
n. the ratio of the breaking stress of a structure to the estimated maximum stress in ordinary use [syn: safety factor]
Wikipedia
Factors of safety (FoS), also known as (and used interchangeably with) safety factor (SF),, is a term describing the load carrying capacity of a system beyond the expected or actual loads. Essentially, the factor of safety is how much stronger the system is than it usually needs to be for an intended load. Safety factors are often calculated using detailed analysis because comprehensive testing is impractical on many projects, such as bridges and buildings, but the structure's ability to carry load must be determined to a reasonable accuracy.
Many systems are purposefully built much stronger than needed for normal usage to allow for emergency situations, unexpected loads, misuse, or degradation ( reliability).
Usage examples of "factor of safety".
But, speaking of emergencies, the power plant should really have a very large factor of safety.
But remember that when you have two strings on a bow, while increasing the factor of safety, you lose the efficiency of the weapon.