In some semantics for conditional logic, a measure of the ‘closeness’ of one possible world to another. The notion of similarity serves to formalize the natural assumption that some possibilities are more remote from the actual world than others, e.g., a possible world differing from the actual world only with respect to some modest historical fact resembles the actual world more closely than one in which the laws of physics differ. To formalize the notion of similarity of one possible world with respect to another involves a number of subtleties, leading to controversial semantic theses such as centring and the uniqueness and limit assumptions.