The propensity or predisposition to be adversely affected. Vulnerability encompasses a variety of concepts and elements, including sensitivity or susceptibility to harm and lack of capacity to cope and adapt.

Vulnerability expresses the relationship between the intensity of an adverse event, the features of the elements at risk (assets, community, system, environment) that affect their behavior, and the measure of the damage resulting from the event (response). Uncertainty in assessing vulnerability is due to insufficient knowledge of the features affecting the response and the possible effects on the elements exposed to an event.

Vulnerability is defined in different ways depending on the types of risk being assessed. In seismic risks, vulnerability is the probability that an element at risk, belonging to a specific behavioral class (vulnerability class), experiences or exceeds a damage threshold (according to a predetermined scale of damage) upon the occurrence of an event of an assigned intensity. In flood risks, vulnerability expresses the expected damage to the elements at risk, the extent of damage ranging from 0 (no damage) to 1 (total destruction).