A "factor of uncertainty" that cumulatively includes all the various imprecisions in assumptions made during the design == "factor of safety".
And in FAR Part 25, it is legally defined as such:
Sec. 25.303 — Factor of safety.
Unless otherwise specified, a factor of safety of 1.5 must be applied to the prescribed limit load which are considered external loads on the structure. When a loading condition is prescribed in terms of ultimate loads, a factor of safety need not be applied unless otherwise specified.