margin of safety

Short definition from Wiktionary

Noun

1. 1
   The difference between the intrinsic value of a stock and its market price.
2. 2
   The strength of a business measured by the amount of income earned above the break-even point.
3. 3
   The difference between a therapeutic dose of a medication and a toxic dose.
4. 4
   The ratio of the strength of a structure to the requirements placed on that structure.