Noun
the ratio of the breaking stress of a structure to the estimated maximum stress in ordinary use
Source: WordNetIt's a very sobering feeling to be up in space and realize that one's safety factor was determined by the lowest bidder on a government contract. Alan Shepard
Emphasis was usually put on the horizontal acceleration factor, for the simple reason that ordinary structures have a built-in safety factor for the vertical component; that is, gravity. Charles Francis Richter