The ratio of a material's strength to the calculated working stress.
The factor of safety absorbs everything the calculation does not model: load uncertainty, material variation, stress concentrations, wear, and the consequence of failure. Its magnitude is a judgement about those, not a number to look up.
Where loads are well known and consequences are minor, factors near 1.5 are common; where either is uncertain, 3 or more is normal. Applying a large factor to a stiffness-limited problem achieves nothing, because stiffness does not fail suddenly.
Using a large factor of safety as a substitute for knowing the load. It buys strength, not understanding, and it does nothing for a deflection limit.
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