1. Mathematical expression of uncertainty applied to data that are used to protect populations from hazards which cannot be assessed with high precision.
2. With regard to food additives and contaminants, a factor applied to the no-observed effect level to derive acceptable daily intake (ADI) (the no-observed-effect level is divided by the safety factor to calculate the ADI).

The value of the safety factor depends on the nature of the toxic effect, the size and type of the population to be protected, and the quality of the toxicological information available.