Generally, a dimensioned quantity which, when added to some other quantity of the same property, accommodates uncertainties in 1) knowing the value of that other quantity or 2) knowing the value of capability relevant to that other quantity. Sometimes, a margin will be quoted as a dimensionless additive value (when it is “normalized” to some reference value of the base parameter). In most cases, the property in question is directly related to a requirement measure.
Contrast with factor (1).